Ask your vet if they carry NeoPredef powder. It's what we use here at the clinic I work at and I've never seen it not work. You'll want to shave the area completely. I would bathe her in a soothing shampoo and let her soak in it for about 20 minutes. If there's any scabbing, gently clean it off, after soaking. After she is completely dry, apply the powder. We usually have them apply it twice a day. Also, you may want to keep an e-collar on her so that she cant get to it and irritate it more. The key is to keep it dry!

Edit:
If your vet doesn't carry it, you can get it from Dr. Fosters:
http://www.drsfostersmith.com/produc...LAID=552563573


Don't feel bad about it - hot spots are very common and can get out of hand before you know about it!