Ah where to start.
Walking into a room where I don't know anyone and everyone else seems to know others. If it's a situation where nobody knows each other, I can handle that just fine. But if I'm the only one that doesn't know everyone else, and it's obvious? I sort of want to curl up into a ball and die. But I usually get my phone out and pretend to text someone (or actually text someone) until I work up the courage to approach someone friendly looking.
Going to the gym and having all amazing runners along side me as I'm huffing through one mile. Yeah that makes me uncomfortable.
Touching chalk. Can't do it, it makes me cringe and gag. Totally out of my comfort zone.
Small talk. I don't like it. I'm not good at the small talk. I will gladly talk to you about things that matter, but small talk? No thanks.
What makes YOU uncomfortable? I probably have more but these are what comes to my mind right away!