My dog is a 10 month old, possible pit bull mix. She is has an average disposition. She has a good diet. Weekly, I brush her. Every two-three months I give her a bath. Monthly, I apply an flea/tick/larva preventive gel between her shoulder blades, on skin-level. Suddenly she has been chewing the hair off her back leg ankles. More and more skin is showing each day. I am really worried about her. Why is she doing this?