My dog keeps biting her back by her tail & has a bald spot now & wont leave it alone what is causeing that

My dog does that to its called a hot spot take your dog to the vet and tell them what's going on and they'll give you dog medicine for it or they'll put you dog on a diet

I agree with Claire. Its called a "hot spot". My dog had one - he would not stop and the area turned a dark grey because it didn't heal from his constant chewing and scratching at it. It's just one of those things - something gets on the skin to cause an itching. The scratching the dog does in response is almost always incessant and obsessive. Honestly, I think the dog is just rubbing the itchy element in more. It should probably be thoroughly washed before the dog gets at the spot. When I went to the vet - she made me a mixture of Apple Bitter with Lidocaine. I means she just poured the lidocaine right into the Apple Bitter bottle. The lidocaine will help numb the spot and decrease the feeling of itchiness- the Apple Bitter deters the the dog from gnawing at it. The spot my dog had was not an open wound (not bloody).