Hello Radar,
I had similar problems earlier this year. Not just a one off though. When the blood comes during the flow may be relevant. I had a cystoscopy as part of the investigation, along with an ultrasound on my kidneys.
The cystoscopy wasn't something I would happily queue up for again, but wasn't as bad as I feared. The cystoscope is designed to do a job and the procedure took between 5 and 10 minutes.
My urologist put me on Dutasteride which cleared the bleeding in about two weeks. I continued to take it for 3 months with no further bleeding.
The urologist wasn't clear what it was, although he could see some radiation scarring.
Hope this helps.
Pierre