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.