Can someone explain to me how this happens?

So about 2 weeks ago I had my lowest weight loss to date. Since then my weight has been kinda stuck around that number. Some days it would be anywhere from a half a pound to a pound higher, but mostly my weight is sticking around that same number. Since I hadn't lost in about two weeks I figured I hit a plateau. Well Saturday night I had a couple of huge slices of deep dish pizza, although by my numbers I still stayed within my calorie allowance for the day. The next morning I weighed myself and it said that I had gained over 2 pounds back. Then when I weighed myself this morning the scale said I was back down to my lowest weight number from two weeks before. So, poof, those extra 2 pounds just vanished over night? I normally don't weigh myself everyday, usually only once a week, but I've noticed this great influx in weight numbers before and I always wonder how it can be. Does anyone know why this might happen?