Hello, I just came on and noticed your post when I was scrolling down. I too have burning in my pectoral muscle muscle into my armpit, and in my arm to my elbow. I have been to every doctor that I have my shoulder doctor has been zero help after a shoulder replacement three years ago. Mine started with burning in the armpit. The neurologist thought it was brachial plexus, but that’s not it. I’m assuming that wouldn’t be what you have either. I really like the idea of an ultrasound. Good luck to you!