Not a painful shit, but my best shit story by far:
I was severely constipated several and hadn't shit for a week, maybe more. Two of my friends and I were at the grocery store buying shit for our trip to Universal Studios, and I picked up a 3/4 liter bottle of magnesium citrate, a stool softener. When we got to the hotel I read the bottle, it said to drink a couple teaspoons to relieve constipation. I drank the entire bottle.
I woke up at 2am, ran to the toilet and took my shit. My entire shit took place within a few seconds and I pushed it out with so much pressure that I flew liquidized shit on the wall 6ft infront of me, all over the shower curtains, and completely lined the toilet bowl, seat, and backrest with liquid shit. I had shit all over my cock and balls, it was seriously everywhere, but surprisingly it didn't smell too bad. I begged my friends to come and look, but they wouldn't, so I took my shower went to bed.
The next morning we went out to the park and after each ride I ran to the bathroom and repeated the process over and over in the public restrooms. Ended up bleeding out my ass a month later and had numerous CAT scans and shit haha, but it was worth it!