why is it that the weekend before my Dr appointments I gain a lot of weight? most of it goes away…

why is it that the weekend before my Dr appointments I gain a lot of weight? most of it goes away after a few days. anybody else deal with this? I've got my glucose test this morning. I'm bringing a sandwich with me to devour after my test lol. also my appointments have officially been moved to every 2 weeks. aghhhh!