Barrie Miskin was newly pregnant when she noticed her appearance was changing. Dark patches bloomed on her skin like watercolor ink. A “thicket” of hairs sprouted on her upper lip and chin.

The outside world was changing, too: In her neighborhood of Astoria, Queens, bright lights enveloped objects in a halo, blurring her vision. Co-workers and even her doctors started to seem like “alien proxies” of themselves, Ms. Miskin, 46, said.

“I felt like I was viewing the world through a pane of dirty glass,” she added. Yet Ms. Miskin knew it was all an illusion, so she sought help.

It took more than a year of consulting with mental health specialists before Ms. Miskin finally found an explanation for her symptoms: She was diagnosed with a dissociative condition called depersonalization/derealization disorder, or D.D.D. Before her pregnancy, Ms. Miskin had stopped taking antidepressants. Her new psychiatrist said the symptoms could have been triggered by months of untreated depression that followed.

While Ms. Miskin felt alone in her mystery illness, she wasn’t. Tens of thousands of posts on social media reference depersonalization or derealization, with some likening the condition to “living in a movie or a dream” or “observing the world through a fog.”