I'm only 20 ... could it really be colon cancer?

I'm a 20 year old female who has had problems with her bowel movements since freshman year of high school. It began as a few years of almost constant constipation (sometimes up to a week!) for which I was prescribed some kind of medicine that either was or was very similar to metamucil. It honestly did not work and it tasted horrible so I stopped taking it. Around my freshman year of college I became close to regular. I would have alot of constipation and then some diarrhea, and then a minority of regular bowel movements. Since high school, I have always had some blood in my stool. Some bright, some dark. I've always had problems with abdomenal pain -cramping, sharp pains, bloating and of course gas. Recently my stool has become small and thin and almost diarrhea-like, and I don't feel like I've finished going. My grandma died of colon cancer and I have relatives who have had polyps, but I'm only 20. I'm planning on getting a colonoscopy or something ....but how likely is it?