Bloody Stool and Abdominal Pain Causes?

I'm a 15 year old vegetarian female. I am fairly active. Recently, (about a week ago) I had a fever that we assumed was due to the flu. But today, I had massive bleeding accompanying my bowel movement and a sharp, strong abdominal pain that seems to be spreading to my back. The blood was a very bright red color (the color you would get from an abrasion on your skin) and had some clots in it. I have had a short history of bloody stool for a couple years now, but never was it so bloody as it is now, leading me to believe that the events are unrelated. Does anyone have an idea as to what could cause this? Any answers would be much appreciated, this is a rather nerve-wracking experience. Thank you.