Your prostate, seminal vesicles, and other muscles in the area will be inflamed by the radiotherapy. It's probably not surprising if they hurt when contracting, although this didn't happen to me during RT - orgasms still felt the same. Ask your radiographers if you can speak with a urologist (they can often get you same day access to consultants), but you might simply have to put up with the pain for a few weeks after RT.
The difficulty to orgasm is caused by reduction in libido due to hormone therapy. You could try some things to increase your arousal, such as sex toys, particularly vibrating cock rings. Some erotica or soft porn might help too, BUT you must both be completely happy with its use or you risk damaging your relationship. Also, make sure all possible distractions are removed - as libido reduces, it becomes more difficult to concentrate on the task at hand, and unrelated things can easily distract you.
By the way, excellent that you've got daily and event dose tadalafil and a pump - you have a good GP.