If your bowel movements are hard they can tear the anus and cause bleeding. Hemorrhoids can also cause bleeding. Youth is on your side, and I truly doubt it's anything serious, but see your doctor to put your mind at ease. I would see a gastroenterologist because this is their specialty. Keep some vaseline on your bottom to stop the irritation. Even a bacterial infection can cause bleeding, there are many benign causes, but see a GI doc to be ont he safe side. Take care.
You could have Internal Hemrroids... I would ck in with my Dr to be sure if that is what you have then he will give you a dipository to get them to shrink down.... Or you could be wiping to hard after a bowel movement causing anal rupture... I would not continue to let it ride be safe and get a ck up....