Why won't she stop chewing her leg?

She may have a hot spot. Depending on where you live, it could be the season. Dog's pick one or a couple of spots on their body and just go at it..until like you said it bleeds! My dog does it to his back and ears. We tried baths, flea preventative, dry skin medicine, EVERYTHING! Finally, we took him to the vet, and they gave us some pills (I forgot what they were called, and for the past 2 years, no more hot spots! I recommend taking your dog to the vet seeing that your story is similar to mines. I hope this helped! Bye!