She probably has worms. The vet will have something.

Unfortunately it could be anything. If it's mites, you need a vet to test and treat her for that. If it's mange, same thing. Chances are good that it's allergies or a hot-spot. In which case a topical spray for hot-spots can be found in any pet store. You need to make sure she can't scratch or bite at it anymore because the longer she messes with it the less it will heal and will more than likely get infected. An E-collar will prevent her from being able to bite her backside but if she can still scratch at it, you'll need to put a shirt or something on her so she can't quit scratching it open.

If you try things yourself to no avail, PLEASE take her to the vet as it really could be a multitude of things that only a vet would be able to diagnose and treat.

Good luck!

My dog went through this as well and I was told it was bacause I gave him to many bathes and his skin was dry so I was told to rub baby oil on him after his bath and it would stop well it worked so no more scratching