I just got back form the vet, she did a scraping. She thinks it might be parasitic. My cats stay indoors, but we have 3 dogs who are inside/outside. So if I find out anything more, I'll post it.

I had a cat (Boo -RB) who lost his hair in clumps - one day it just started falling out over his hips - no scabs, no ringworm, the vet found nothing wrong with him at all. It fell out and never grew back - the vet said it might be from stress. That is why skin problems worry me.