Most likely it’s post-radiation “obstructive voiding symptoms” (OVS). A common side effect in the first month after radiation and usually resolves fairly quickly. Although Flomax mainly affects prostate cells, it also can help relieve stricture in the bladder neck.
Here’s a study:
https://www.ncbi.nlm.nih.gov/pmc/articles/pmc4118163/#!po=38.3333Good luck!