The side effects you mention are all completely standard when the bowel gets exposed to radiation. The bowel wall gets irritated, and it assumes this is because of a bowel infection (which it isn't). This will initially give diarrhea, but that will subside into just excess mucus, and that will probably reduce and mostly vanish. Stools can change substantially - when the diarrhea subsides, rabbit dropping type stools are common initially (lots of tiny stools).
Side effects on the bowel tend to lag about 2 weeks behind the treatment, so you should expect peak symptoms about 2 weeks afterwards, and then it will start to subside. While you have diarrhea, reducing fibre in your diet will help. I had to cut out all fruit, veg, and brown flour until a month or two after radiotherapy, and then I could gradually reintroduce them. The mucus farts (as my oncologist refers to them) had gone by about 10 months, but farting was safe again well before this, in that I could sense again if a fart was going to be safe or not - that sense is lost during radiation, but like most of the other things, does usually recover.
This is the result of stray radiation when treating prostate - if the bowel is treated directly as in your case, then the side effects may be more extreme. (Bowel cancer patients may have more relevant experience.)
At a local support group in the last week, the topic of excess rectal mucus came up, and several of us a year or more out from treatment commented that we do still have more than we did (not to the extent of mucus farts anymore), and this does make it easier to take a dump than it was before the prostate cancer.