I've experienced the same symptoms as the original poster and it's been on and off for about 20 years. After orgasm or stool, a slight feeling in my bowels conected to my rectum to start, then a gradual increase in discomfort, and depending on the episode gets so painful I break out in sweat and think I may puke from the pain(havent yet!),cycle is about 30 minutes give or take, like the one I just had

..(awkward when it happens at work), then it gradually decreases. The only thing I found that alleviates any of it is sitting on the toilet, but even that sometimes isnt enough, I try and pop some ibuprofen as early as possible and drink lots of water.
I had a lower GI about 6 yrs. back, and it so happened I had an episode before I went in, (I had wanted to go one last time before the test), what they saw was an irritated colon(as you'd expect!) that led me to a colonoscopy which found nothing. I've tried to nail down what causes it, it can be months, and even longer than that in between bouts. I used to think it was MSG in Chinese food, which a few times I have eaten the stuff and within a day had the pain. I've also thought it could be related to a bad combination of being a bit dehydrated and sometimes that triggering it,(which Chinese food is so high in sodium it could all be somewhat related). In general I try to stay hydrated as much as possible, right now I'm trying to think back about what I ate and did not drink to cause such a painful one today, some Boston Market(salty stuff) and not much water last night.
I'm hoping it's that I have a sensitive colon that can be triggered if I'm not careful and nothing else and it continues to be manageable in that way, sure sucks to get a painful one though. Sounds like this is one of things that should be checked out and if all the tests hit a dead end, raised again if the symptoms and regularity changes, I was hoping to find some concrete information, but I suppose it helps just as much to know that many others have had the exact same issues with no concrete issue as a cause.