I've hit a plateau...help!

I dont know whats going on, on most days I'm under my calorie goal or just on it. Ive had no more than a handful of days in a 7 week period where I've gone over, and not by much. I work out between 2-3 times a week.... spinning, running, etc... mostly cardio. I lost three pounds in three weeks, so it was going well... about a pound a week the way my plan is supposed to work... and then I just stopped losing.... at first I thought it was water weight from female water retention due to that time of the month .. but that has passed and a few weeks later im still holding steady at 3 lbs lost. What do I do? Has anyone else experienced this?

Thanks,
Erika