Good to hear this isn't unheard of... been lurking here on-and-off for months since my diagnosis back in Aug. I had my LARP on 11/21/11.
For several years, I complained off and on (about 5-6 times a year) of a pain in my prostate after ejaculation. It most often occurred after bowel movements within 20-30 mins after ejaculation. I frequently had UTIs and was originally diagnosed with bacteriological prostatis, due to reduction in elevated PSA (4.5 down to 3.0) following a month of levaquin (and my age... 43 at the time). Turns out I was very early stage, but I did have an enlarged prostate (about 45cc). Doc said I may have had the infection along with the cancer.
Since my RP, I've been able to achieve orgasm (with no erection) without much problem (although I note that I've had at LEAST "10 different versions" of orgasms in this past month or so since the catheter was removed).
Anyway, since the RP, I've noted after several orgasms, I've had the EXACT same pain in the area where the prostate used to be (like Snowbound). Not quite as intense or as long-lasting, but same general feeling and location. It's like someone pinching that area real hard for about 3-4 seconds; similar to a muscle spasm or charlie horse feeling. Prior to surgery, this pain might last as a throbbing/droning "pinch" for about a minute.
I rationalized that it was probably the nerves dealing with the rectum/bowel moving into that space where the prostate was, but it was a bit unnerving, since it was the same feeling before the prostate was removed. BUT, I also noted that I had a HORRIBLE problem with an (mostly) external hemorrhoid due to constipation following surgery. I'm thinking this issue may potentially be something with internal hemorrhoid issues. (Still investigating that).
I'd be interested to hear if any of you experiencing this pain also noted any problems with hemorrhoids.