The crawling Shimmer in the mirror

"I was sitting in a chair in some kind of salon, or maybe a school nurse's office, hard to say, and my head started itching, small at first, then not small. scratched and felt something move under my fingers, tiny, quick, and when I pulled my hand away there were specks on my nails, dark, wriggling. Someone was standing behind me combing through my hair, a woman I couldn't see properly, and she kept going quiet in a way that made my stomach tighten. I asked her how bad it was and she just said "hm" and kept combing. Comb came away full every time, no matter how many passes. I tried to get up and the chair felt stuck to me somehow. I saw my own reflection in a mirror across the room and my hair looked like it was moving by itself, a slow crawling shimmer. I wanted to scream and only managed a whisper. Woke up clawing at my scalp, hands shaking, checking my fingers in the dark."