If you've gone an a very active holiday for a couple of weeks, and eaten less than usual, or less calorific foods, maybe you've lost some body fat. It's possible. Another possibilty is the change in diet has your digestive system less bloated so you can do up your belt tighter. I know if I eat more than normal in a single meal, like an all you can eat buffet, my belt feels extra tight, and it's just bloating, which goes away the next morning.