What is it called when you get physically tired in social situations but when you're alone you have a lot of energy?

Anxiety defense: Provided there is nothing physiological going on, I wonder if being tired in social situations is a defense against anxiety in social situations. Ask yourself, does the mere thought of crowds raise your anxiety? How do you experience crowds? How long have these reactions been going on? These questions are best explored with a competent psychologist, after medical phenomena have been ruled out.