Don't EVER hesitate or wait to talk to your doctor or parents about weird things you find on your body. The longer you wait, the harder/more painful it could be to fix the problem.
Disclaimer: I AM NOT A DOCTOR! This is NOT medical advice, only information on anal cancer!
I was 38 at diagnosis & that's considered young. I haven't heard of anyone younger than mid-30s and it's usually diagnosed in people over 60. A-C is primarily caused by HPV, but you don't have to have receptive anal sex to hit the A-C jackpot. It's also very rare (we don't even get a ribbon) in that around 5080 men and women (fairly equally) are diagnosed every year.
My radiation oncologist told me it takes 10-15 YEARS to show up as a tumor, although pre-cancerous lesions can be detected and removed earlier IF your doctor does a thorough exam. Therefore, it would be highly unlikely, bordering on a medical miracle, for anyone in their teens or early 20s to have anal cancer simply because of the length of time it takes for a tumor to form.
It is far more likely that a significant lack of fiber in your diet is causing you to be constipated with small, rock-hard, stinky bowel movements that you strain to pass. The strain then pops out a vein near your anus & bingo, you have a hemorrhoid.
My "journey through anal cancer" (what a stupid saying) has taught me that a proper poo should be soft but so much that it leaves streaks on the toilet. It should be a deep brown, rather banana-like in shape/size and while it does smell, it does not clear the room or make you gag for fresh air. Normal BM frequency is 3x per week up to 3x per day.
If, after changing to a proper diet, your symptoms continue--ESPECIALLY if you have bright blood in your poo, seek the advice of your physician immediately.