When the vet put her on science diet, did he tell you which ingredient that he was trying to avoid.
What were you feeding?
I feed Royal Canin
Here is a link to their online book on the yorkshire terrier specific food.
http://www.royalcanin.us/dogfood/yorkshirebook.html
This next link has other links that you can click on for more information. The video is worth watching just to see the yorkies.
http://www.royalcanin.us/dogfood/yorkshire.html