am I developing selective mutism?

I’m 21, and have usually had no problem talking to people I know, but have always struggled to make conversation with people I’m meeting for the first time if there isn’t an initial click. I think that’s pretty normal, though. Anyways, recently, I’ve begun having moments where it feels like my teeth are sort of merged together, and the sheer amount of effort it would take to open my mouth and speak is so daunting that I would rather not. This hasn’t happened while I am around people, rather more when I am alone, but I have declined friends’ phone calls because of it. I’m going to bring it up to my therapist next week, but does that feeling resonate with anyone on here?

For any background, I have pretty bad anxiety that manifests itself as physical tenseness a lot. I struggle with constantly tense muscles and aches because of it, and I’m only ever entirely physically relaxed when I am asleep. I’m wondering if this could be a symptom of the anxiety? But I’m also wondering why it would take so long to present, since I’ve had anxiety for most of my life but this feeling is relatively new.