The issue you might be having now is that you may have had so many food restrictions with UC, suddenly introducing a bunch of new foods into your diet is causing a bit of havoc with your system. Your body wasn't used to processing those foods and has to learn all over again.
Have you tried Beano and other digestive enzymes? They can help. You should also be taking probiotics if you are not already. It's also very important to try to introduce foods back into your diet very slowly and gradually. If one food isn't working for you, eliminate it for a while. You can always come back to it later on as everything else starts to settle down.
Many people find keeping a food diary useful in figuring out what foods they can/cannot tolerate. Basically, you record what you've eaten, what time you've eaten it, and then any symptoms you might be having as they occur. That way, it's easier to trace back to a potential problem food. My dietician suggested this when I had my ileostomy, though it can certainly work equally well for a j-pouch. While I certainly wasn't meticulous about keeping one, I jotted down some general notes in the early stages and it did help, even just to keep my mind at ease. The ileo was so new to me that I expected all sorts of disasters any time I tried something new, and I was happy to see that for the most part, I felt okay.
Another thing, it does help to think outside the box. You may want to focus on what you are eating WITH the carbs, rather than the carbs themselves that might be causing the issue. For instance, if you're eating spaghetti with tomato sauce, it might actually be the tomato sauce causing the straining, frequent bms and butt burn. Tomatoes are very acidic and can cause discomfort. Same goes if you're eating potatoes heaped in gravy. It could be the gravy and not the potatoes that's causing the issue. Just food for thought... pardon the pun.
I hope you can get it sorted out!