Since how long in the past, did the symptoms begin? Does the pain go anywhere? So, is it only during sneezing that you get the pain? Out of the scale of 10, how severe is the pain with 10 being the most severe and 0 no pain at all? Any history of left shoulder dislocation or musculoskeletal trauma in the recent past? Do you have any medical history of any chronic illness? If everything is ruled out with investigations, it could be musculoskeletal spasms, that must go on its own. Consult your doctors for more details/examinations/investigations TO RULE OUT ANY SIGNIFICANT ILLNESS please.