This may be musculoskeletal pain related to the way you are sitting. Consider whether you are slouching to one side or using the left arm in a way that may be straining chest and back muscles. You should try to get up and stretch regularly when you have to sit for a long time, and make sure your using a comfortable chair that supports both your lower and upper back. You can also try Ibuprofen (Motrin or Advil), and if the pain continues you should see a doctor.