Have you tried Calmoseptine? It works for me for light soreness near the rectum associated with a BM. Unfortunately for me, only opiate pain killers or overdoses of Tramadol work when the pain ramps up past the mild stage.
One of the things that the large intestine does is to recycle acids, so your stool is more acidic now as it exits. Not to mention that with many people with a pouch, the BM exits under more pressure than normal, creating a friction "burn."
There is also the possibility of cuffitis and nerve damage, but those things would not be normal for most people.
One of my problems, I think, is that I used hydrocortisone cream for years before my surgery, when I had UC. I had not heard of Calmoseptine, and it was the nonly thing to relieve the stinging and itching in the rectal canal. Unfortunately, it thins the mucous membranes and makes the nerves closer to the surface. It should never be used as I used it. Stupid mistake. I think that is a major aspect of my BM-associated pain, but my Dr. disagrees.