What can cause shoulder pain in front of the clavicle on the left side when sitting having back support? and no pain when standing or laying

Unclear: I'll say that "in front of the clavicle" close to the breastbone is the sternoclavicular joint. The pectoralis major muscle attaches to the "front" of the clavicle, but i've never seen a strain of that muscle hurt in that location. Other small ligaments and muscles attach to the clavicle, but i just can't imagine anything that causes pain only when sitting.