I have experienced something like this in the past. I watched my Mom's Pomeranian at my house a little while back. He has been a family pet and loves me and my husband; BUT while he was at our house without my Mom, his hair fell out and he actually went bald in a few spots, he stopped eating, and lost one pound in a matter of a week (this may not seem like a lot of weight but when you are only 7 pounds to begin with...). Everything seemed to be a big "effort" for him and he acted like a feeble senior dog the entire time we had him. He is only 6 and doesn't normally act like this.
Once he had been home for a few days he resumed his normal day-day behaviour, and acted like himself again.
I think for some animals being away from their families is a really stressful event. This isn't the case with my dog however, I could leave him with a random strange family and as long as they fed him he would be happy