Heavy menstral??

So I've always have had abnormal menstral (about once every 6 months) this year it has been once (sometimes 2×) a month last in 4-5 days, this month was the 2x a month only on Christmas morning we had a "quicky", and then later on I started extremely heavy flow with huge clots, lasted heavy until the next day, the 27th and today has only been light when I wipe, but I still feel cramps and nauseated, can some one tell me why?