Very heavy flow with extreme pain

Hello,
Im 20 years old and normally i am irregular. I am currently on the rag. I am having extreme pain and I have almost passed out due to how extreme the pain is. my abdomen is tender to the touch. When I started The bleed was like a brownish and it was like I was spotting. Now I have to change my tampon every hour or sooner(I am using super plus kotex tampons). I am also having VERY large blood clots when i go to change my tampon, the clots are almost black but still have a slight tight of red. the clots are bigger then a silver dollar. Whats does this mean. I have never had a flow this heavy.