could also be spasms in the prostate...I've had this happen before even without having intercourse and while painful my doctor described it as spasms in the rectum or prostate and not serious...
nonetheless, I am not a doctor...I strongly suggest you see yours to be certain.
Jim
Could be from an enlarged prostate. Ejaculation makes the prostate "pulse" from ejaculating it makes the prostate get more inflamed which could be the feeling you are having.
this doesn't sound like an STD problem, have you had your colonoscopy? your suppose to get one I believe. Altough I'm not sure if it is 40, 45, or 50 when you're suppose to. I would talk to your doctor about this and I wouldn't wait.