Hope ibrox is still here, I didn't see this until I was looking for the proper forum to ask a different question.
There are a couple of possibilities, but one that stands out because it happened to me is developing CPPS. This is a muscular issue of the pelvic floor muscles where they are too tight and can lead to a bunch of pain and other issues. The pain can pop up after orgasm/ejaculation because the pelvic floor muscles have just been contracting for the orgasm.
Stress can be an issue, as can other triggers (in my case it's pain in the pelvic area that can set it off).
I'm here, and I am most appreciative of your thoughtful response. I read the entire wikipedia piece and any other material I could find, and the more I think about it the more I am leaning towards a stress-related cause and effect. For me, it is not really a chronic situation. It might only occur 3 or 4 times a year, out of maybe 50 or 60 sexual encounters. When I think back, it may only have happened once in the last year. I used to have trouble orgasming, and there may have been an element of "desperation" entering into my efforts. That would seem to set the scene for the tensing of the pelvic floor muscles, resulting in the pain immediately after orgasm.
I never did mention this to my GP, and, believe me, I am very vigilant about reporting any unexplained ailments. Because the pain went away in a few minutes, and I experienced no other symptoms, I was inclined to treat this as "one of life's little mysteries".
Thank you again, @moliere. Much appreciated.