As Maisym00 said, get it checked out. It could be a whole load of things - piles, ulcerative colitis, Crohns, an anal fissure, anusitis, gastritis... At your age and given that it has been going on for so long without too much escalation, I would say anything TOO serious is probably unlikely.

I just had a colonoscopy for rectal bleeding yesterday - turned out to be piles, which was what I suspected really - and when I saw the specialist, he said at my age, the odds of cancer would be less than 1%. Now, I am nearly twice your age and the odds go up with age, so I would suspect the odds are even lower for you. That said, it's best to get it checked out. It's likely something very minor, but just in case, it is anything more serious (which does not mean cancer or anything on that level, but things like IBD), best to get it looked at.