Chris, as already said it could be an infection. Urethral strictures after RT is a known issue. I had a Urethral stricture after surgery. A cystoscopy may be required to find the problem. On occasions a camera and guide wire were required to get a catheter in. If you go into retention don't hang about, backing up urine into the kidneys is not ideal and retention can be uncomfortable. The Catheter may have to stop in until a resolution is found, and post treatment could be in for 2-4 weeks.
You can monitor your flow by timing and measuring your urine output. Mls/ seconds will give you an average flow rate.
Looking at you previous posts looks like you had 20 RT sessions in April 2020.
Hope you get sorted quickly.
Thanks Chris