What causes anal bleeding after defication? when should i see doctor?

Could be: Could be a anal fissure or hemorrhoids. See your doctor to confirm.

Depends: At your age , usually hemorrhoid bleed especially if it is on the tp! if you r constipated( round or hard stools or straining when you go , not how often) you can get hemorrhoids or small cuts in the skin of the area called a fissure that can bleed too. Fissures are more likely to drip a bit into the toilet . If the problem is not constipation definitely get checked out real soon by your doc.