I'm baffled by my body sometimes. A week ago, I stepped on my scales and hit a new low weight (202.4). Then, because clearly someone

was out to get me, I steadily gained a pound a day for the past five...5...FIVE days. What the heck? I went over my calorie goal most of those days, but only by about 200-300 calories, which should still be way under my maintenance needs. (I do 1200-1300 per day usually because I'm pretty sedentary.)
Thankfully, I stepped on the scale today and three pounds had miraculously vanished, but I still haven't made up the difference to get back down to 202. What's up with this? I was expecting to be knocking on the door of onderland right about now. Can this much weight gain really be water retention? What else could cause it?
It's making me crazy!!