How old was she when you got her? Puppies learn bite inhibition from their mother and siblings, but if she was taken from her mother too soon, she missed this and other valuable lessons in socialization.
You will have to teach her bite inhibition yourself. Here is some information:
Dog Owner's Guide: "No bite!"
Just a reminder to everyone looking for a puppy.....Yorkie puppies should be twelve weeks old before going to their new homes. The Yorkshire Terrier Club states in its Code of Conduct:
8) All puppies leaving the breeders possession will be a minimum of twelve (12) weeks
of age to facilitate adequate socialization as well as appropriate emotional and
temperament development through interaction with siblings, dam and other dogs.