One morning last August, Lili Trifilio was feeling emotional.

“I’m honestly so nervous,” the singer-songwriter, then 24, admitted, her voice rising as she shook her head. It was the day before her indie-rock band Beach Bunny would headline a sold-out show at the Music Hall of Williamsburg in Brooklyn. Beach Bunny’s recent success had seemed abstract to Trifilio, since most of it had happened during lockdown, on the internet, but the group’s biggest New York show to date would make it tangible.

“Over the pandemic, Beach Bunny has grown like 200 percent,” Trifilio continued, between sips of an iced Nutella mocha latte at a cafe not far from the venue, “and I don’t know what to expect.”

Trifilio has a wide, toothy smile and a choppy bobbed haircut that she likes to dye different colors — magenta, lilac, rust — though that day it was a naturalistic blonde. Onstage, she’s known for her bubbly, earnest positivity; at a recent Beach Bunny show, she gave an enthused recommendation for a local vegan restaurant, urged the audience to get their Covid-19 booster shots and led the entire crowd in singing “Happy Birthday” to a fan. On albums she’s known for the emotional lucidity of her songwriting, which seems to trap fleeting feelings in shimmery amber.

Much of the recent growth in Beach Bunny’s popularity came via “Cloud 9,” a bouncy, guitar-driven love song from the Chicago band’s February 2020 debut album, “Honeymoon,” which in March 2021 became a viral hit on TikTok. Over 360,000 videos have now used Trifilio’s lilting valentine (“But when he loves me, I feel like I’m floating/When he calls me pretty, I feel like somebody”) to soundtrack photo reels of their lovers, crushes and besties; it has racked up more than 240 million streams on Spotify.

Several fans have asked Trifilio to record an acoustic version of “Cloud 9,” so they can use it as their wedding song. Trifilio finds it all a little ironic, given that she wrote it in the final days of a failing relationship.

“The lyrics are so smart,” Tegan Quin of the indie-pop duo Tegan and Sara said in a phone interview, “and melodically I find all their songs to be really creative.” She and her twin sister Sara were fans before “Cloud 9” took off, but the song’s popularity provided an opportunity for them to collaborate with Beach Bunny on a new version — as requested by fans — that also features “she” and “they” pronouns.

Beach Bunny’s music has plenty of admirers outside of the TikTok demographic, too. The actor Bob Odenkirk discovered the band several years ago while flipping through The Chicago Tribune, and he “immediately dug them,” he wrote in an email, because he found their sound to be “connected to the indie rock that I loved from the days of yore,” like Pixies, Sebadoh and the Cavedogs. He’s since become a vocal fan and even made a cameo in Beach Bunny’s recent “Star Wars”-spoofing video for the song “Entropy.”

“I’m an older white guy, and her lyrics are about longing and written from a female perspective,” Odenkirk added. “But I still feel very connected to the pain and estrangement of my 14-year-old self, and I always will.”

While the breakout of “Cloud 9” (and a prior TikTok success, “Prom Queen”) brought the band opportunities, Trifilio feared being pigeonholed or not taken seriously. “I was such a crab about it,” she said, twisting her straw. “Like I’m going to fall into this genre of internet bands. I was like, ‘No, I want to play big stages and play with bands I like, and not be thought of as cringey. I had all these weird ego dilemmas.”

Perhaps to combat those fears, during the pandemic, Trifilio taught herself about music production. She watched YouTube tutorials and countless interviews with producers who inspired her, like the electro-pop star Grimes. When the band started recording its second album, “Emotional Creature,” at Chicago’s Shirk Studios last spring, she felt more empowered to experiment.

“I think it’s cool that she’s an all-in-one show and does everything hands on,” Trifilio said of Grimes, citing her aggressively upbeat 2015 single “Kill V. Maim” as one of her favorite songs. “So after listening to her talk about production, going in I was like, ‘OK, I don’t really know how to do this, but can we make the beginning have this vibe? Before, I never knew to bring in those references.”

That increased ambition is apparent across “Emotional Creature,” out July 22, from the bright, explosively catchy leadoff track “Entropy” to the thrilling, nearly six-minute finale, “Love Song,” which in its satisfying final moments weaves together a medley of several other songs from the album.

“It still sounds like Beach Bunny,” Trifilio said, “but it just sounds a little more grown up. Which I’m happy with, because I’m growing up.”

TRIFILIO WAS RAISED in the suburbs of Chicago, and she started taking guitar lessons with a friend in fifth grade. “We did not have the attention spans for it,” she said with a laugh in a recent video interview from her childhood bedroom, where the purple walls matched her tie-dye butterfly shirt. (She moved into her own place during the pandemic, but still visits her parents frequently.) “But it was fun. That’s where I learned my basic skills. We were just like obnoxious kids, and so after a couple of years I quit because I had other things to do as a 13-year-old.”

Later in her teen years, Trifilio started participating in neighborhood jam sessions and teaching herself cover songs. She has noted on Twitter, amid the occasional Hannah Montana quotation, that while journalists compare her sound to “cool” ’90s bands, her most direct influence is the pop group Aly & AJ’s 2007 album, “Insomniatic.” (I hear traces of the alt-rock mainstays Letters to Cleo and the cheery indie-pop group Velocity Girl.) When she was 18, she thought, “Well, I’ve learned a lot of covers. Let’s see if I can use this combined knowledge to write something.”

The result was “6 Weeks,” a wailing, melancholic recollection of heartbreak (“Let’s begin at the end, when you tore me apart”) that she recorded on her computer with just an acoustic guitar. She presented it to her guitar-lesson friend as casually as possible: “I was like, I made this song, and I’m so embarrassed. Can you listen? I think I’m going to delete it.”

Trifilio’s pal gave her a much needed confidence boost — and an ultimatum. “She was like, ‘I’m going to stop being your friend if you don’t put this out,’” Trifilio recalled. “I was like, whoa, OK. Stakes are high.”

For the next few years, while she was studying journalism at DePaul University, Trifilio continued writing sharp, hooky power-pop songs and uploading them to a modest but growing online fan base. In 2017, she also started playing shows with a local group of guys — the drummer Jon Alvarado, the guitarist Matt Henkels and the bassist Aidan Cada, who was later replaced by Anthony Vaccaro — and her solo project became a proper band.

Trifilio’s candid, plain-spoken lyrics often sound like internal monologues; sometimes they’re pep talks, other times they give voice to her demons. The title track from the 2018 EP “Prom Queen” straddles the line between the two. “Shut up, count your calories,” it begins over a jangly chord progression. “I never looked good in Mom jeans.” The song became one of the most downcast tracks to inspire an internet dance craze. As her anxiety builds, the song becomes a critique of aesthetic perfectionism and diet culture that Trifilio, who has admitted that she has “struggled with [her] own body image,” knows all too well.

Many listeners related to Trifilio’s unabashed presentation of her insecurities. But “Prom Queen” found success on a platform that often rewards young people for adhering to the very conventions Trifilio was critiquing. Some noted the irony when the popular TikTok creator Addison Rae — the app’s honorary prom queen — posted a video of herself dancing and grinningly lip-syncing to a song that goes, “I was never cut out for Prom Queen.”

TikTok can make a song incredibly popular overnight; it can also very often divorce a song, or even fragments of a song, from its larger context. Trifilio, who was not yet familiar with the app when “Prom Queen” blew up in 2019, was concerned that listeners who only heard a line or two of the song might misconstrue it as an endorsement of behavior like calorie counting. So she pinned a lengthy statement to the song’s YouTube video, clearly stating her authorial intentions.

“I wrote this song for every person out there that has felt insecure, unloved, or unhappy in their own skin,” she wrote. “Please don’t harm your health or well being to live up to these invented expectations, it is not worth risking your life over.”

Three years and another round of app-fueled success later, Trifilio said she’s learned to relinquish control of how her songs might be received. “You know, I use music in the same way,” she said. “I’m sure artists had different intentions than how I interpret things.” “Prom Queen,” she added, is “kind of the public’s song now.”

AT A JULY 2019 show in New Mexico, Trifilio was surprised to notice a familiar face at the merch table: Odenkirk. He mentioned an upcoming audition he was preparing for, and as they parted Trifilio wished him good luck. “He spun around, gave me the finger guns, and he was like, ‘I don’t need it,’” she recalled with a laugh. “And I was like, ‘That’s right, you don’t need it!’ I need that level of confidence!’”

The bold and self-assured sound of “Emotional Creature” shows how far she’s come. Sean O’Keefe, who produced the album, called her “one of the best songwriters I’ve ever gotten to work with, and I’ve been fortunate enough to work with a lot of really great songwriters.” (His credits include Fall Out Boy and Plain White T’s.)

On the new album, piercing pop-punk tunes like “Gone” and “Deadweight” challenge emotionally ambivalent partners to wear their hearts on their sleeves. “You’re a diamond/Wish you could see you the way I see,” Trifilio sings on the mid-tempo rocker “Weeds,” during a chorus that offers loving advice to a heartbroken friend — or perhaps the singer herself. Writing the album, she said, helped her to confront her history of “shame around feeling big emotions.”

“That was, like, a therapy moment,” she said. “‘Wow, you have a lot of shame around being an emotional person, even though every human has feelings.”’

Trifilio has since come around on TikTok, too. “There is definitely a young girl audience, mostly coming from TikTok, with very little experience of even attending shows,” she said. “They tell me, ‘This is one of my first shows,’ and I’m like, ‘That’s amazing. I hope you go to so many more.’”

Such experiences seem indicative, to artists of a previous generation like Tegan and Sara, of a palpable change. “Streaming has devastated the music industry for artists, but it’s also made it really easy to be popular in corners of the industry that just didn’t exist when we were coming up,” Quin said. “Beach Bunny is an example of that. There’s just this vibrant, incredible scene flourishing around them because people can find them.”

At the Brooklyn cafe, Trifilio had noted, “When I was 16, there would be some band I’d see and I’d think, ‘It would be so cool to be in a band.’” Preparing to greet some of her new fans in the flesh the following night, she added, “It’s amazing to think that someone might come to a show and maybe that inspires them to learn a Beach Bunny song on guitar. And then they learn other songs on guitar. That’s wild.”