My treatment was surgery, not radiation, but I can relate to passing bright red, bloody urine twice during the two years after treatment. The cystoscopy is a good idea, because bleeding like that can result from other causes in the urinary tract and bladder.
It could have been a small kidney stone, or the cause may never be known, but you shouldn't ignore it.
In my case, the uro also had me undergo an ultrasound of the kidneys.