Welcome to YT!
I'm wondering if it could be a food allergy. Have you taken your pup to a vet - hopefully it could something as simple as changing her food - what are you feeding her right now? Some good quality foods are Orijen and Go! Now. Many grocery store pet foods are really lacking some of the basic necessities - Yorkies eat so little it isn't too expensive to feed them really good food. Hopefully someone on YT will have a yorkie that has experienced the same thing your little one has. Good luck and let us know how things are going!