John, that is something I have experienced too.
For me the cramps are relieved by going to the toilet and it is clear there is wind as well as poo. I deduce they are just trapped wind, I seem to have reduced my ability to pass wind separate from poo. But things have got better. I am about a year ahead of you, radiotherapy (salvage in my case) finished end August 2022, and this now hardly ever happens except first thing in the morning, possibly associated with the change of posture from horizontal to vertical. That is a time when I am always within easy reach of a toilet!
I am sure Francij is right and a short term fix would be to move to a low fibre diet. But I prefer to stick to the food I like (including a fair amount of fruit, veg, and wholemeal foods) and it seems my body now copes pretty well OK.