What could cause pain and rectal bleeding while pooping?
The most important question is that have you had colonoscopy? At your age it is very important to make sure that this is not rectal cancer.
The most common causes of your symptoms are anal fissure, anal fistula, hemorrhoids, colitis (inflammation of the colon), anal warts, anal cancer and several other diseases.
I think you should see a colorectal surgeon to examine the anus.