I had same issue but occurred later in process. Radiologist said it’s from inflammation of the tissue caused from radiation. Take advil or another NSAID like Advil. It helped me quite a bit but didn’t totally solve the issue. I would take 1 every 4 hours during the day. ( You have to take a lot over a long period of time to damage kidneys so I didn’t worry about taking it for 2 months or so) Problem went away about 3 weeks after treatment completion)