How can i gain 5 lbs in 1 day?

i check and double checked and sure enough i gained 5 lbs in one day. how can that be. i'm on my monthy. could it be water gain? this is so mind blowing. here i thought i was doing good.. man what a bad day...has this happend to anyone else.. please i need your help..

We greatly appreciate your interest in helping us build our Q&A database. To discourage vandals from posting garbage, however, we require people to register before posting.