Hi fellow Bottoms!

So I've noticed something over the years as I've bottomed, and I'd like to run it past y'all. For context, I'm a 35yo thembo (AMAB), full time bottom.

When I'm getting fucked, especially when the Top is doing long strokes inside my hole, I'll notice the muscles along my anus will start contracting slowly, like a build up of pressure and tension, before it starts spasming and pulsing, usually from my core outwards towards my anus. It's not painful by any stretch, quite the opposite, but it can be a little intense. The Tops seem to enjoy it, describing it as if I'm milking them. The pulses tend to last a few seconds, and then everything relaxes. So far, I've counted 18 times in one night, with multiple Tops (stopped counting after that, stopped really thinking even, lol).

It doesn't sound like a prostate orgasm. What is this?