Mathematical law of reverse weights and measures

So, I've already admitted that math/numbers is not my forte, but this just baffles me: I fought the battle of the bulge since last September when I got serious about nutrition in an effort to lower my cholesterol. The Formula goes like this: there are 3,500 calories in a pound. My goal was to have a negative Energy Balance of 500 calories a day, resulting in a weight loss of 1 pound/week. (energy Balance is the number of calories consumed minus the number of calories expended.)  I managed, through cutting out (and I mean really cutting out) desserts/sugar, ramping up the fruits/veggies, and burning over 500 calories/day in rigorous exercise, to lose 14 pounds in 7 months (more math: that's 2 pounds/month or a reasonable, if less spectacular 1/2 pound a week). Then yesterday, I decided to eat a cookie....(ok, maybe 5 cookies...but I'd been deprived for so long and these were sooo good!) And...you guessed it! Gained almost 2 pounds in one day!! And the Universe laughed. By the way, if anyone is interested in that cookie recipe, I'll share the love.