YEOUCH! I feel for you Ariel! I've had that in passing, it never has lasted more than a day or so, though. I've always figured it had to do with my overall state-of-the-moment .... if I'm a bit "flarey" I hurt all over, and that includes every hair on my head. And yes, mine has thinned, but I am happy enough that I still HAVE it!
I hope it goes away quickly -- maybe try a gentle shampoo with some tea tree oil in it? Stay out of the sun as well - that may contribute.
Gentle hugs -