This is one of the side effects that happens from radiation. It is not real common, but it does occur with a lot of people.

Primary Treatmentn Adjunct and salvage radiation possible side effects
Radiation proctitis
Rectal issues
Rectal bleeding
Bladder infection
Breaks in bones radiation damage to bones
Fibroses in bladder which can reduce capacity

In general, toxicities after postprostatectomy radiation using photon-based techniques have been tolerable, although the rates of late grade 2 and gastrointestinal (GI) and genitourinary (GU) toxicities range from 10% to 20% with image-guided intensity-modulated radiation therapy (IMRT).

What this means. “ late grade 2 and gastrointestinal (GI) and genitourinary (GU) toxicities”
Above describes moderate symptoms of damage to the bowel and bladder, requiring minimal intervention but impacting daily activities, and occurring after salvage radiotherapy for a recurrence or persistent cancer after initial treatment. Gastrointestinal (GI) symptoms can include moderate diarrhea or bleeding, while genitourinary (GU) symptoms might involve increased urinary frequency, pain, or intermittent bleeding.

A link to article
https://pmc.ncbi.nlm.nih.gov/articles/PMC8019576/