I've always been really shy, but lately I've wondered if it is selective mutism. For example, when I join a work call I can't ever just say 'hi' unless it's a 1:1 meeting. I have a weekly call with 4 colleagues and I can never just say 'hi' first once I've joined. If they say hi, I'll say hi back, but if they don't I won't say it. Sometimes I've been in calls and not uttered a single word, which I feel awful about afterwards, but I just can't do it. There's not really a nervous feeling or anything, it's like my brain just won't let me speak unless someone invites me to by directly asking me.
Sometimes there will be things I know about and know the answer and I just can't speak and say it. Sometimes when making small chat and something is relevant to me I just can't say it. For example, if my colleagues ask if anyone is going to the office on X day and I am, I can't say it. If they then say '[my name], are you coming to the office on Tuesday?' then I can answer.