Hi, hope you are doing ok. I saw this post while searching for an explanation for symptoms just like yourself. Finally someone experiencing something similar to myself.
Not sure if you have managed to get a diagnosis yet but I am still in the dark. I will make a follow up appointment with my GP but am not particularly optimistic.
Like you I have experienced what I believed to be prostate pain on an off for years (decades). Like you said it comes on and lasts for about 20 minutes during which time I have to lie down in a foetal position and wait for it to pass. Very hard to describe like the prostate being squeezed. Varies from 5/10 - 9/10.
Also does happen after ejaculation sometimes.
The last couple of times this has happened severely it has been followed by passing blood from my bowel some time (hours) after. This then passes.
As a result I went to the GP and both times got referred for a colonoscopy. Happily they found no cancer or polyps but there was some inflammation. So the current thinking is colitis but I don't have a clear diagnosis. Also saw a urologist who didn't think it was a prostate issue.
I can go for many months without any issues so can live with it but would really like to know what it is and why it happens.
Anyway thought I would let you know you are not the only one and hopefully you have had a diagnosis.
Best regards.