How did the pain started. Was there any injury or jerk.
It may be due to soft tissue strain of Rotator cuff muscles in shoulder.
Next Steps
You can start with hot fomentation, take analgesic with muscle relaxant tablet, apply analgesic ointment locally. Physiotherapy including Local US, SWD can be started for few days.. If the pain still persists, we may have to plan MRI cuts of Shoulder region as well to rule out any soft tissue compromise..
Health Tips
Avoid heavy work or lifting any heavy objects with left hand..