Krissi said to realise that weight fluctuates day to day, within about five pounds or so, and this is totally normal. Don't worry that you gained fat overnight because what you ate, how much you drank, whether or not you've gone to the bathroom, or how much you've been exercising can all affect your weight — so don't obsess about the number on the scale.
Monitor your weight by how you look and feel. Take weekly or bi-monthly pictures to ensure you're maintaining, because the scale numbers can go up, but your body could look leaner and more muscular.