FR,
It sounds as if you have had a recent medical workup, so it is probably safe to say that you are experiencing chronic and persistent physical symptoms of anxiety.
Tension, improper breathing...these can cause the thoracic muscles to become tight and fatigued.
Breathing exercises are pretty key in relieving this problem. You may be anxious breathing without even being aware of doing it...
...and as MW mentioned, we often don't
feel anxious....but the subconscious is aware of the stress we may be suppressing!
See the Resources for some good instructions for breathing techniques...or let me know you are interested, and I'll post my favorite for you here.
S.C.