This happened to me a few nights ago.
A couple of nights ago me and my guy friend (who we will call Joe) were hanging out in my dorm room, just him and I. He was watching some stupid videos on youtube and laughing his ass off and I was attempting to read for one of my classes, but I felt extremely horny.
I was laying on stomach and just sort of kept slowly but strongly thrusting my pelvis into the mattress. Good thing Joe was concentrating on the funny videos and didn't realize what I was doing. So I just put my head down, my arms serving as sort of a pillow, and kept basically just like channelling energy to my clit, if that makes sense. I was getting really, really into it and I didn't think it was possible but all of a sudden I started orgasming without ever being touched by myself or anyone. It was amazing and was definitely the strongest orgasm I have ever had.