Heavy bleeding and large blood clots?

Okay, so it started three days ago as just a light bleeding. Then it progressed and got heavier and heavier to where I have to change a pad every hour or it might soak through. And I would pass large blood clots. Before this I was experiencing tiredness, nausea, headaches and dizzyness for a bit. I was due for a shot of depo in August and never went cause my hormones were out of wack. It was normal bleeding until a month ago that my period lasted 2 weeks stop for a week and went for two weeks again. And then it stop up til three days ago. Between that time it stop was two and half weeks. Now it's different, cramping are extreme, passing large blood clots and small ones sometimes but large usually. Very heavy bleeding. I have appointment tomorrow, and I just wanted to know if anybody else experienced this, and if you had, what was the cause of it?