Could be everything tiggersmom said but it could also be a food allergy. My rottweiler had a so called hot spot on his hind end at the base of his tail. We took him to the vet and ruled out mange, infection, insect bite and any other parasitic thing known to dogs. We spent a few hundred dollars and found out he was allergic to beef by products of all things. Put him on a beef free diet and a month later the hot spot was gone and his coat grew back in. Just some food for thought. He was much happier too.
Chances are she's going to need antibiotics... Here's a site you should read over... www.thepetcenter.com
She needs to have some benadryl and maybe a cortizone shot to clear this problem my dog had he same problem and that's what the vet said.
It could be any number of things, ringworm, mange, an infection from some sort of bite. You need to take her to the vet, and get her looked at to make sure that she's going to be o.k. Hope this helps, good luck.