Alright so here's a bit of my back story.

I've been on birth control for six years (I'm 18), because I had my period almost every day for about 6 months straight.
It took me a few tries to get a good pill that worked well for me.

I also have a few fibroids (cysts/non cancerous tumors) in my uterus, one is pretty big but not big enough for them to want to remove it.

Now,
The other day, I had 1 or 2 pills left
And got massive cramping.
Went to the bathroom and I was clotting very badly
Probably the size of a golf ball, maybe bigger.
I've read online how some women get their period even on the pill
But it's never happened to me in the 6 years I've been on the pill.
And I've never had them so heavy. And it isn't getting less heavy.

I'm also starting to get pains in my thighs

Does this happen to people a lot or should I be worried about my fibroids and whatnot?