20 [F4M] CA.. Feeling like a f*cking slut

To be honest, I don't even know where to start with this.
I'm terribly humiliated and scared. We were just hanging around in his room, watching YouTube videos and talking about school and other things. After a while, the conversation turned sexual, and he grew a little stiff. I thought that was funny, so I teased him and told him it was small (which isn't a negative thing). I don't know what he was thinking, but he proved to me that it's not small.
To be honest, I was pretty interested, so I started feeling it and almost gave him a handjob. We just stared at each other and said, "What the hell did we just do?" when he got there. I never thought l'd be in a scenario like this in real life... Now that I'm back in my room, I don't know what else to do but write it down.