I had something like this happen to me years ago in my teens. My older sister had one of those exercisers with a belt on it. The kind that had the motor about waist high and then a belt about 3" wide that goes around your belly. The idea was it shake your middle section and the fat was supposed to go away. I have no idea why she had it, but she did, she was always slender.
So, one day my family was away, I was alone in the house all day, and for me that always meant a huge hardon and enemas, lots of play!
So, I decided a fun way to start would be with this exerciser to just jerk me off. So, I dragged it to the bathroom and squatted down on the belt. My idea was that belt really vibrating, and it had strong vibrations, on my very hard dick would be a good time. So, I got started with it. It felt really good. Eventually squatting on the belt and it vibrating back and forth it eventually worked its way off my dick and groin into my asshole crack right up against my asshole. And I was sitting on the belt hard, very hard. Being gay, but not even knowing it at the time, it felt extremely good as I played with my asshole all of the time. And this was really fun, all of that intense motion against my asshole.
As it went on I really had the belt up hard against my asshole and as I noticed later my underway was really brown when I had finished, The last thing was I did not want to get poop on my sister's exercise machine! LOL! What a give away that would have been as to what her little brother was up to when they were gone.
So, I squatted harder and harder on the vibrating belt, by then my hardon had gone away, as I was totally focused on my asshole, and I mean totally focused.
I had no idea what I was doing, and no idea about an anal orgasm or anything like that. What I noticed was a stirring started in my rectum, kind of like when getting sort of close to cuming. The stirring feeling continued and I was just enjoying it all. Suddenly, I felt my asshole contracting on its own, really contracting hard, then it started convulsing very strongly, and I felt my entire rectum contracting and releasing, almost like the speed when cuming and it was very very intense. I was amazed, the feeling was incredible. It seems it lasted a couple of minutes. I turned off the exerciser and then ran over the sink had an enormous jerk off session. Those contractions were very strong in my rectum and anus, even into my groin area, very strong and tingling.
I never had that happen again. I tried it again after that, I just could not get it to happen again. I must have been at just the right angle such that it was shaking my asshole and my entire rectum. The organism feeling was deep, not just in my asshole, but all the way up, it was incredible.
The closest I can get to it anymore is estiming with a bipolar butt plug and doing ghost fucking. It's of course not the same, but gives me a good feeling. I am very very anal, so now I use ghost fucking to get my asshole off.