My dog growing up did the same thing. It turned out that she had allergies. Talk to your vet to see if it could be allergies. Also, what kind of food are you feeding her? Did you just change the food? She could also have fleas. They like to nest near the bum and head. Just above the stub was where my Kilo was always effected by fleas. And Dakota chewed himself raw in the same area when Patience (stray) brought the nasty freeloaders home with her.