Its a hot sensation. I had an ex introduce me to it. She was sucking my dick one time and pushed a finger into my ass. I lost all control and came right there. Hard.
I love the feel of my anus. I don't get a lot of pleasure from penetration, at least haven't yet. I had the same experience as you, and went over the top as well. It was out of this world. Now, she will do it every once in a while, and it's a great treat.