Birth control does this. I have been using the depo for the past 8 or so years. If i stop using it i may not get a period for 4 + months or i get more than normal. It can take up to 2 years for your cycles to return to normal once you stop birth control. Its always worth getting it checked out by a doctor if your worried though. My experience with this type of birth control is that it plays with my cycles A LOT and goes from one extreme to the other until my body gets itself back to normal.
Just see your normal doctor and see what he thinks, it could just be one of those random things that happens with us sometimes, or it could mean a few other things. Just see your normal doctor for advice. It could be something like endometriosis or a girlie condition like that, but I wouldn't get too worried at this point :)