Just a thought, but the only thing I can think of right off is it could be the food you're feeding her or possibly fleas?? I know mine are allergic to grain and chicken, and it makes them scratch a lot and my yorkie bites her paws. I switched to a grain-free and chicken-free dog food to see if that helps them.   
I'm sure someone will be along soon with the experience and knowledge to help you out.   
Also, welcome to YorkieTalk!     
				__________________    Hi from Oklahoma City from me and the girls: Holly, Lacey & Tinker Belle      |