Not sure I can help diagnose, but my kitty is having poop issues.
Mine cries every time it comes upon him, and he starts to scratch and run. Its like explosive diarrhea, basically it comes on so fast he cannot make it to the litter tray. What might help is putting the kitty locked in a room, with a separate litter tray. I have 1 in the basement, and 1 upstairs. So when he panics, he has one close to him at all times.
You could also try a fluffy rug..weird..I know..but if he cannot find either tray he instinctively goes for a rug, I found when he is really upset he uses our fluffy bath mat - i guess it soothes his red hot bum
This might help at least control the poop issue until you can get a diagnosis.