8month old terrier mix keeps chewing on her hind leg and foot. it has been going on for several weeks if not months..the skin on her paws doesn't seem to be hurt or irritated. i see nothing in between her toes. ive looked all over for some kind of rash or something..can you tell me what may be happening with her and what i need to do to make sure she is ok? ive never had any other animal do this.thanks very much heidi