Why do I have a pain in my tush when I need to have a bowel movement?

Hemorrhoid / Fissure. Could be a hemorrhoid or fissure. If you are constipated then i would increase your fiber intake and use an osmotic laxative such as miralax (polyethylene glycol). If perists see your doctor.
Before? During? Can't tell here. Could be anal issues like fissures (tearing pain during defecation), hemorrhoids, rectal prolapse, inflammatory bowel, narrowings (strictures), tumors, cancers, gyn issues, other. Need to get examined. See your doc, .