It must be that PMS monster, it's known to do things like that.
Hang in there, drink a lot of water, and those extra pounds should whoosh away soon! 
You can't gain more than the weight of your food... like if you eat a whole pound of bread, you can't gain more than a pound, it's physically impossible. I guess it's possible that the sugar crash made you want to eat more than you would have otherwise, in addition to water retention from PMS.
