Help!!bleeding problem

I am now 17 years old. I started to have blood in my faeces when I was 16. It may because I was stressed, I drank not enough water that I need to force out the faeces with some effort. I have also read that it may due to hemorrhoid. However, I found shockingly large amount of blood in my faeces in most of the cases. It's like I am having menstruation. The amount of blood is so large that the blood can hide my faeces in the toilet bowl after egestion. I cleaned with tissue paper and it is full of blood. The blood is not dark red but colour is of the "fresh" blood. I think that's mean I have no bleeding in my guts. Then that's conclude I may have bleeding at my rectum. I worried that I may got cancer in my rectum of any other parts of my alimentary canal. Or I may get some serious problem that related to hemorrhoid. Yet I always wonder why I am still young but I will get these kind of problems. What should I do?