What causes anal bleeding after defication? When should I see doctor?

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.