I had a similar issue. My period last month was VERY heavy, and I'm usually pretty light, although I am on the Pill (sadly). For example, I began eating primal and my new pack of pills around the first of January. Not even two weeks into to that pill pack, I had a period heavier than a normal period - and I had just barely gotten over the previous one. It was a doozy! It last almost a week. Then I got my "normal" period about a week and a half later, a little earlier than expected, and same flow. So I had two huge periods in one month. I was NOT A HAPPY CAMPER.
I looked into reasons for this phenomenon. Some ladies on the "low carb friends" message board said they had similar issues. I can't explain the science behind it this late at night, but from what I understand, this isn't unheard of.
Of course, if it gets really painful or unbelievably heavy, I would call the doctor .. I did just to let them know what was going on.