Is your cat on flea control meds? It could possibly be flea bite allergy even if she gets bit by only ONE flea. Cats and dogs will chew themselves raw in that area for several reasons. Could be impacted anal glands. Only way to know for sure is to take your cat to a vet. Don't wait too long or it could turn into something worse. Good Luck!
I had the same thing happen to one of my dogs--itching, scaling/scabs, hair-loss on the tail and haunches--it turned out to be a condition caused by skin mites: "Demodectic Mange." To diagnose it, the vet had to put scrapings of the skin onto a slide, stain them, and then you could actually see the mites/eggs under the microscope. One treatment (injection,) and a few follow-up pills, is all it took to get rid of them for good. Simple if you know what to look for, I guess.
