I am also an anal cancer survivor (13 years ago!). I was treated with chemo and radiation -which resulted in severe burns. I noticed a change in my ability to control my bm’s immediately but rarely had accidents. About 5 years ago, I started having many more accidents and dealt with almost constant diarrhea. I was sent to a GI for an assessment. It turns out I have been diagnosed with Exocrine Pancreatic Insufficiency. I have no idea if it was related to my cancer treatment. However, I started taking Creon, which is actually derived from pigs. The creon has almost entirely solved my problem - though I do have “flares” every few weeks which I can control by increasing my Creon. The medicine is very expensive but the manufacturer has a patient financial assistance which I have qualified for.
I’m on a couple of EPI forums and it appears that not all doctors embrace the dx AND it can be confused with other digestive problems. I have not found anyone on the forums who has a history of anal cancer so it may not have anything to do with the cancer treatment. However, the radiation burns had affected my sphinctur so I assume it at least made the EPI harder to deal with.
You might ask doc if EPI could be at least contributing to your challenges. Good luck at finding help!