"She's shy."
I've heard that my entire life, I heard it just a few minutes ago. But I don't think that's what it is.
When I'm introduced to someone, I automatically say "hi" and then I physically cannot think of anything else to say. My brain will not let me.
And if I do happen to remember what other people might say, my mouth won't let me say it. I feel flustered, like its a weird thing to say, and my vocal cords Will. Not. Let. Me. Even though it's a completely normal formality.