Ahhh, I know what you mean! I do the same thing! I don't know for sure, but I'm guessing it's because your body is gearing up to lose a lot of fluids, so it keeps as much as it can beforehand. It sure is a pain though. On the plus side, I try to be extra good and push extra water that week, and then when the TOM has flown the coop I often find that I lose an extra pound or so, probably in water weight, but it's fun