This was happening to me too. I saw a doctor, and they referred me to a physical therapist.
The numbness is possibly from a pinched nerve in your shoulder. The weight of your arm when you are at the computer or otherwise sitting up, causes the pressure and then the pinching or inflammation of the nerves in the shoulder.
What helped me?
I had an ergonomic study done. I bought a computer chair that supports my elbow/arm weight while not forcing my shoulder to support the weight for long periods. The difference was like night and day. I had less discomfort within two days and within 10 days I didn't have another bit of problem.