ams...... I experienced similar as yourself for a couple of days after takedown; thought I had a golf ball sized blockage, really uncomfortable and painful; created the sensation that I needed to go all the time
At the time my surgeon said it was due to the two skin types meeting, one within the anal canal on the inner side which can tolerate the bile acid produced by the stomach and the skin on the outer side but still slightly within that was never designed to.
Applying a small amount of ilex protectant paste onto the tip of my finger and applying it to the inner side of the anal sphincter brought instant relief.
A few days after returning home, the issue occured again, still does from time to time but not as bad; on each occassion I apply the ilex paste and I've even pinpointed an area within where my discomfort actually occurs.
I discovered that during takedown my anal canal was dilated and I believe that during the stretching process, the skin within the anal canal tore, creating open cuts and such wounds are normally called fissures.
Fissures can also occur when straining during BMs and although I'd not passed a BM in the traditional sense for many years, when it was time to, it's a natural sensation to bear down but such straining is not required with a J pouch, it's more a relaxing of the muscles and let gravity take its course.
If anal fissures occur, during each and every BM, the open wounds are being immersed in bile acid.
In a few weeks time the bile acid produced should subside, so will bathroom visits and once you've returned to a normal diet the acid produced won't be so acidic.
I've never had to endure the tradition diaper rash type butt burn, only what I've described.
I hope this helps.
