I was diagnosed with high-functioning autism in conjunction with bipolar disorder (and OCD, which seems like a facet of the autism to me, but whatever)...

Things finally make so much more sense to me.

When I'm in a manic period, I'm charming, gregarious, words slide easily out of my mouth and usually in the right order. People love me, and instead of feeling uncomfortable, I mostly feel the adrenaline rush that comes with jumping out of an airplane or something. It's exciting to me, even if it is scary.

Sometimes in the same evening I'll shift from that to being exactly how OP described. I never even notice the mood shift, but I notice the anxiety creep in. The uncertainty, and the gradual sensation that this isn't exciting, something is deeply wrong and in my exuberance I've allowed myself to become so much more immersed in things than I am okay with.

I've tricked myself into somehow being the center of attention, and now people want to talk to me, and engage me in conversation, or follow-up on earlier conversations. It's all way more than I can handle, and I end up feeling like I'm having a heart attack, with my chest pounding and a familiar headache creeping in at my temples.

And I can't. I can't handle it anymore. I was falling out of the plane, and it was great, but now my 'chute didn't open.

So I usually run away. Literally. I just walk away from everyone and go stand alone outside for a while. Long enough for my heart to stop pounding, for my head to hurt less. It lets me rehearse how to talk to people again so that when I go back in I have a cover story for my absence, and I start to figure out how I can make up excuses to leave.

Sometimes I drink. Sometimes I drink a lot. If nothing else, it seems to give people an explanation for my erratic behavior, and as a bonus it makes me care less about basically everything.

I leave the night feeling as though it has gone terribly, and that I'd have been better off not going and drawing attention to the fact that I've got the social skills of a paranoid Mitt Romney. Discouraged, I end up going to bed resenting myself for trying, but stubborn enough to insist that I'm going to do it again even if I end up mostly hating it, and consequently being frustrated with myself.