Why does it hurt when i poo ?

Im 13.when i go toilet to poo im sometimes scared, i sit there and try to push and it fells like a needle is picking me through my ! (IT HURTS SO BADLY !) and then when i wipe my bum , blood comes out (bright red). Right now i haven't pooped for around 5 days ?? and i kind of feel like doing poo and i went to try but it hurts so badly that i just ignored it ,.... ADVICE !