What Causes Earlier Period And Huge Blood Clot?

I am on my period which I wasn't even suppose to get, very painful and had a huge blood clot come out that was like more Jelly then liqud, it was red with black and white in it.... still pain but took A LOT of medicine to ease it by now but feel sick to my stomach. I'm 19, about 5'7 I guess? and about 115 pounds. I've also had a lot of abdomen pain lately but nothing as bad as tonight.