Please Help!! Could I really have gained 5lbs water weight overnight? Wedding coming up!

I gain 3-5 pounds between morning weigh in & lunch!

If you didn't eat 17,500 calories above maintenance in that period of time, you didn't gain 5 real (fat) pounds. I've seen a jump of as much as 5 pounds from one morning's weigh-in to the next, and you better believe I didn't eat 17,500 extra calories! 5 was unusual in that length of time, but 2-3 is absolutely common.

And I certainly vary in weight by several pounds over the course of a day, depending mainly on what I've eaten & drunk, but not eliminated.

If you're tracking your eating meticulously, and you know you didn't eat above maintenance, don't worry about it. It'll drop off. Just stay on a healthy routine.