i have no injury and no reason for muscle problem yet doctor says it must be muscle related or a byproduct of acid reflux. the pain is only when i breathe deeply and its on left side. I thought it might be my heart because started with achyness in the arm. That went away overnight Its going away slowly. My energy is low because I have to shallow breathe. No colds, etc. I am 78 yrs.,.and am active. %-)