Some water goes away when you pee....but when you retain water you retain it in your body not in your bladder. If you gain overnight or lose overnight it's water weight, you can not gain fat overnight. Constipation can also add weight....I think I read somewhere it can add 5lbs or even more depending on how backed up you are.

Good news is even if you gain fat if it comes on quickly chances are it'll go quickly too. New fat cells are not established in the body yet and are the first to go when your eating right!