Pretty sure this happens to anyone with anxiety disorders. Tensing up your muscles is a natural human reaction to physical danger. Your back muscles and shoulders probably get tense too, you just don't notice it as much.
It is perfectly natural to tense up under physical fear, but our anxiety makes us tense up under fear that is irrational and not real. It is most likely when your face tenses up, you are in no physical danger at all, whatsoever.
I like to tell myself to "loosen up" when this happens. This simple phrase reminds me that there is no rational reason to fear, and that this tensing is only an irrational reaction by my anxiety, which makes me perceive physical danger that is not really there.
Here is an excerpt from my CBT about this:
"If you're in public or in a social situation and you're feeling
anxious, just remember these two words... "Loosen Up"
What happens when we're anxious is that our body and muscles tighten up
automatically, many times without us realizing it. Technically, our
physical body is preparing to meet a threatening challenge that really
isn't there - but, since we think it is, our muscles become tight and
rigid.....
By realizing this and just LETTING GO of the tension in your muscles by
"loosening up" - you are calming yourself down and making the situation
a little easier for yourself."
-Excerpt from, Overcoming Social Anxiety: Step by Step, by Dr. Thomas A. Richards