Early very heavy period

Hello,
I am 22, and until now have always had a fairly normal menstrual cycle. I am in a monogamous relationship, and although I am no longer on birth control, we always use a condom.
Sometimes (although not everytime), I've had some spotting during sex. (I have no idea what the cause of this is.) The other night, it happened... but then it basically turned out to be my period. This wouldn't be strange except that if it is in fact my period, it is almost 2 weeks early!! On top of that, it has been extremely heavy compared to my normal period.
Does anyone have any suggestion as to what might be causing this? Anything I've read says that early periods are often caused by stress (which I really am not experiencing.. at least not until now!) or a severe change in body weight (which has not occurred), or menopause (which is unlikely considering I'm 22!).
Any help would be very much appreciated.