could be different things from stress to different birth control maybe ex: if you recently got on the pill or changed to a different one. But to be save and make sur etheres nothing medically wrong with you, maybe get checked out by you doctor?
How far apart were these "2" periods. last period I was so light that I had what seem to be 2 periods. However when I looked at the calendar I realized that there was just 1 day in between my so called "2" periods. Looking at the calendar I'm certain now that I didn't have 2 different periods just one that was so light that I thought it ended earlier then it really did.
With that said I agree w/LittleBirdFly and get yourself check out by the doctor.