I experienced the exact same thing recently. You can read the discussion here:
http://www.3fatchicks.com/forum/showthread.php?t=167020
I looked through my calendar and noticed a trend. Despite all my efforts, on the 2nd week of Feb and 2nd week of March I gained weight... and it was incredibly discouraging! Then I read a little bit about ovulation and water retention. Ovulation typically occurs after 11 days from the start of your last cycle. This was exactly when I gained, but I was back down a couple of pounds the following week.
If the timing makes sense, maybe ovulation has something to do with it. Maybe not. Like most everyone else mentioned, it can be a lot of things... hormones, sodium intake, muscles retaining water after exercising... Just don't let it get you down. Keep up the good work. You're doing a great job! You'll see a loss sooner or later.