Horemone imbalance most likely. Have you tried birth control? That usually regulates mine.
I would go see your doctor, This could be due to a number of things. Are you on the pill? as that can cause this Other things such as thyroid problems, hormonal imbalance, an infection of some kind ( not just an STI) or even stress can cause this, best thing to do is see your doc though :)
have u tested for pregnancy? whether u did or not i think u shud contact ur dr. u might have a cyst or have a complicated pregnancy...good luck :)