Rectal bleeding...help..?

So ive been having occasional rectal bleeding when I go to the restroom for a couple yearsbut i just went to the doctor for it yesterday. He did an exam and tested me and said that because of the blood that he wants to do a colonoscopy. Im kind of concerned because colon cancer runs in my family on my mother's side and my grandfather dies when he was in his early 40's because of this. My mother had some trobls with palyps a couple years ago. she was in her mid 30's. Im 21 years old. What could cause this?