Why...

Okay, so after a week of strict calorie-counting and daily workouts I got on the scale anticipating some sort of loss, if even half a pound. No, I GAINED 2 1/2 pounds. So all last weekend (a three-day weekend, mind you!) I ate what I wanted and laid around on my ass. What happened? Come Tuesday I can take my jeans off without un-buttoning. Where's the logic in this?