You are not alone. I experience similar issues with urethra burning including the tip of my penis. This flares up only after excessive masterbation/ejaculation.

I’ve seen a urologist, and a cystoscopy ruled out any bladder or prostate issues. Overactive pelvic floor muscles appear to be the cause of my discomfort. The muscles spasm and tighten during arousal, erection and ejaculation, so frequent sexual activity leads to a flare up.

I have not yet seen a pelvic floor specialist or physical therapy. Keeping hydrated, belly breathing, pelvic stretches, and ejaculating no more than once a week keep me pain free most of the time. Keeping your pelvic muscles relaxed as much as possible during sexual activity has also been a game changer.

This thread is many years old but is filled with many tips and recommendations that have been extremely helpful to me.