Water weight - especially when you start eating less, you can lose several pounds of water weight, but generally our weights can fluctuate from day to day by several pounds.
Don't be too surprised if you gain some of it back, and definitely dont' expect to continue losing at that rate.
Starting out, you lose a lot of "water weight." The first ~5 pounds that come off are exceedingly easy, and a lot of people get unreal expectations based on how easily those ~5 come off. Don't get too excited, so that you don't later get discouraged! Just remember, slow and steady. Check this article: http://www.fitsugar.com/What-Does-Mean-Lose-W ater-Weight-14968229
Sometimes you have a big week, on AVERAGE over the course of you jourey it will probably end up being between 2- pounds a week, if you average out the losses over the amount fo weeks you have lost. Not all weeks will be big either.