Your vet will surly know what is going on, but my first thoughts would be a mild kidney infection or UTI. Lucy gets UTIs when under stress, if we don't keep her parts clean, and sometimes from change in diet. My jack russell was diagnosed with kidney problems around 5 years ago and that was how she was acting, but was also peeing in her sleep. Because of her kidneys, the last time we went away I hired a friend to stay in the house. When we got back Tia had a kidney infection, tonsillitis and a high fever. I thought she was going to die. I had actually hired someone to stay in the house with the dogs. Then found out she kept them in a separate room, gave them no attention and when I went into the room the water bowl was bone dry! They would have been much better off at a kennel where at least they would have had the entertainment of other dogs. You can also inspect a kennel better than you can someones heart. Anyway, sorry to natter on -but, I figured you would already be worried if you thought it was her kidney's, so I just wanted to tell you that my jack who was so sick, was just fine with TLC and antibiotics. And antibiotics clear Lucy's UTI's right up.
I guess I should have started a new thread called worst pet sitter experiences.