That isn't it

Blasted sore spot has kept me awake most of the night

. If I touch the buttock it doesn't reproduce the soreness. DH has looked closely with his jewellers' loop, nowt of note.
But it is SORE to touch: knickers, sheet, towel. Maybe a small ingrowing hair? Or a really really tiny spot? Or something trying to escape from within

?
Answers on a post card. Please.