How To Cure Bleeding From My Rectum?

I have been bleeding out of my rectum for the past few days. I am a 19 year old female who is generally healthy, with no major health issues other then a reptured appendix when I was a child. I do not engage in anal sex. The blood starting fairly light, but is now so heavy that it is filling the toilet and pooling in the bottom of the bowl.My bowl movement are fairly regualer and I have not been straining to have a movement. I have pain(similar to the dull ache of menstral cramps) in my lower back and left side. What could this be?