It is most definitely water weight. That is the only thing that can make your weight do that at that drastic of an amount. Always try to weigh yourself first thing in the morning after going to the bathroom, this is your most true time, due to the fact that you are usually pretty depleted at this time. Try not to get too hung up on the scale though, go by the mirror, how your clothes feel, etc. The scale will drive you crazy!
-------------- The one goal you will NEVER achieve is the one you never attempt.