How can I gain 10lbs overnight?

I have been on a strict diet for about 3 months because I was almost 50lbs overweight and it was affecting my health. Basically weight watchers/calorie counting/increased activity. I lost about 8lbs the first week, and a steady 2 a week since. However, every once in a while I let myself eat something I really want, in the past 3 months I've had 2 days I let myself eat what I wanted. First day I had 3 slices of pizza for my birthday and half a personal bag of chips later that night. Next day I woke up and weighed myself, I had gained 8.lbs. Over pizza and some chips. That was a month ago. Yesterday I had one serving of Kraft mac and cheese I made with tuna. I woke up today +9.1lbs from yesterday. I thought the pizza was a mistake, it had to have been, but it happened AGAIN TODAY!!! How is this even possible?? I didn't even eat a pound of food let alone almost 10!