I have been suffering from straining since my takedown 6 months ago now. I want to start off by saying I didn't have a stricture, I have cuffitis. But here's a couple things you might try to help you out. One is to buy one of those stools you can raise your feet up so you are in a proper position. Have you tried different positions and do they help? Another thing is warm water. Soak in the bathtub up to your belly for a few minutes or run warm water over your abdomen and buttocks while on the toilet.
Also, are you still on narcotics? I was on narcotics for a long time after takedown and that may contribute to being back up. If that's the case you may want to tread slowly with some forms of laxative. Don't overdue it cause butt burn is just as bad.
Remember you are still 2 months post surgery, so foods with fiber and bulk can back you up. What's your diet like? Be very careful with things like popcorn, vegetables, and nuts. Especially if you have a stricture. That may be contributing.
Also, remember you've just had multiple surgeries and the pelvic floor muscles aren't the same. You may need a round of pelvic floor therapy to correct the fact that some of the muscles might not be working like they should. Some people tend to clench more after these surgeries for fear of incontinence. That can contribute to straining.
As you can see there are a lot of factors. Also it takes time for the J Pouch to function well. I'm NOT a doctor, but my guess is that there's some foods or narcotics potentially contributing to that stricture, and that constipation is becoming painful. Post again with all the details of your diet and meds.