I have pain below my left breast and mid-back on left side. This pain only occurs when I sit for a prolonged period of time. It never hurts when I am standing or lying down. What would cause this pain?
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.
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