Oh yeah, this happens a lot. I just broke a 6 month plateau and lost 2.5 pounds last week, but this week I took a mid week weight in and I've gained about a pound of waht I lost back - i was distraught, but i realized it is PMS - this time I am going to work out right through my period - I tend to cramp slightly, so i wont do anything super intense, but just keep the rhythm going.
The good thing about PMS weight is that it comes right off after periods.. i hope that's the same with you guys?