Subscapularis muscle strain is a commonest cause of severe pain in left shoulder and left upper back region. Subscapularis muscle is one of the rotator cuff muscle of shoulder in both side, which helps in rotation movement of both shoulder. However there is also possibilities of other diseases. One among those diseases is very serious issue. It is called Pancoast tumour of lungs in left sided upper lobe. Pancoast tumour is malignant in nature involving surrounding tissues, vertebrae and ribs. Most common variety of pancoast tumour is non small cell lung cancer.
So, it is very important to first consult oncologist to rule out chance of tumour or cancer in lung. You may take analgesics with antacids for pain relief. Take complete bed rest for 3days and avoid heavy exercises or heavy weight lifting. Physiotherapy may be started after pain relief. Take Care.