Hi,
The stomach can be sore as there is a big wound in the middle that goes right in through the muscle. If you lift heavy items or are too active you might strain it and it does let you know.
I've never heard of a rectum being sore and don't know why although perhaps you're straining or not drinking enought too keep things soft. The wound on your bladder needs most careful treatment and straining on the toilet is not to be recommended, maybe you aren't anyway.
The urine problem could be an inflamation although could it be related to something you're drinking or doing. It could be something you could ask a chemist. Perhaps drink more water.
Of course you could ask the surgeon or ring Macmillan, especially if you have a named nurse. You could also ring a Prostate Cancer UK nurse. That's what they're there for. The surgeon might put one of his doctors on to call you.
Feel free to bounce your thoughts around here. People reveal things they wouldn't normally. I know I do as I haven't told anyone I know which I sometimes weaken on but haven't.
All the best
Peter