I don't have many periods (on Seasonale) but I just went through one and I was amused. I didn't gain... I just didn't lose anything. And then WHAM, 2 days after my last day my scale dropped 4 pounds. Considering I lose 1 a week, that was HUGE! Then I went back up 2 a couple days later, which is about where I should be at my normal rate. I managed to
at the whole thing.
Try to not take the scale TOO seriously. I know it's hard (is for me) but a couple days are not the end determiner of how you are doing. Just remind yourself that it's not possible for you to have actually gained weight. That is, unless you really did eat an extra 3500 calories recently.