Radiotherapy can cause late onset (> 3 months after treatment, and sometimes years after treatment) bleeding in any of the areas targeted, including prostate, urethra, bladder, and rectum. This is usually harmless unless it's sufficient to make you anemic. If you take any blood thinners, this increases the chances.
These cases do always need to be checked out though, just to be sure they're harmless.
Blood is bright red for a few days after a bleed, but as the blood cells disintegrate and the iron in them is released, it literally goes rusty, and hence rust coloured. I have come across a couple of cases of men who didn't ejaculate until months after a biopsy, and were extremely alarmed to find they had black semen.