Why does my bum hurt when i poo?

Fissure, piles, PNS: Anal fissure is a split in the anal skin and quite painful when skin is stretched by a bm. Hemorrhoids can certainly be pailful as a pilonodal sinus or cyst. Suggest you try preparation h for temporary relief then see your dr, or a colorectal surgeon, they only recommend surgery if absolutely necessary, if pain persists. All this is assuming you aren't constipated. Good luck.