Help!

Anyway, my periods were very regular for a while. Every 28-30 days like clockwork. About 5 months ago, they started to change to every 38-40 days. After two months of that, I spoke to my doctor. He told me not to worry as long as I had no other worrisome symptoms and as long as it kept this schedule. I might also add that I always had cramps with my PMS but never anything severe that was not controlled by heat or Midol. 

So, my next  period is not supposed to start until January 4th. But this morning I had excruciating cramps, so much that I got dizzy and nauseous. And I figured I had just eaten something bad and was having tummy troubles. But then, a couple hours later, I began bleeding A LOT. I've always had a heavy flow and have used ultra absorbency tampons and they typically last 4-6 hours for me. But I was bleeding through them every 30 minutes today. All day. It's only slowed down within the past few hours. My cramps cannot be controlled and I'm very nauseous. 

What could be happening????? Anything helps.

Thank you.