Hmmm...that is strange that the vet didn't find anything wrong. What's the next step? Just watch her? I was going to ask about her hip, too. I had a horse one time that came up lame one day, and after many, many tests on his leg they found out the injury was way up in his hip. I know horses aren't even close to yorkies, but that's the first thing I thought of when you described it.
I hope she feels better soon!