I don't think it will be permanent either. A few weeks ago, I had another yorkie here with me for a week and my ZoE had a horrible time. Turned into a totally different dog. Once the other yorkie left, she was right back to her usual loving, secure self.
Just keep giving her lots of extra love & reassurance during the time you're babysitting. The other pup needs extra loving too, as I'm sure it's stressful on the visiting pup as well.
Best of luck
