Hormonal fluctuations can be a main reason for this. Many things can affect your cycles. If you're having it, and it lasts more than a week or so, then see your doctor.
i cant give you a definite medical answer on that, however, i can tell you i had the same issue in 2009 and my gyno performed an ablasion and completely stopped my menstrual cycle.. I was diagnosed with cervical cancer and i definitely recommend you see your gyno. Women can never have enough pap smears of check ups.