I've mentioned to relevant people a couple of times in the past that I generally feel like I am "acting" in most social situations. Like I have an idea as to who/how I need to be in that moment and act accordingly. I'm used to it and it's not really something very conscious in the moment, but if the event lasts a long time it gets kind of draining since you have parallel tracks going - reading the room, predicting what reaction someone expects at any time, issuing that reaction, etc. It's not that I don't enjoy the events, but I can "fill up" on interaction quickly and want to go home early to recharge.

I figured that was just being an introvert.

That's more like social anxiety.