Two possibilities

This can happen if you have hemorrhoids even if you don’t strain.

And then there is radiation proctitis which is more likely
Yes, blood in the stool three months after prostate radiation can be a problem, often a sign of radiation proctitis, where radiation damages the rectal lining, causing fragile blood vessels to bleed, but it's common and usually manageable with doctor guidance, though it needs checking for severity to rule out other issues and determine if treatments like stool softeners, diet changes, or procedures are needed.