Pain below neckline, shoulder till elbow

I have been experiencing pain internally from the lower portion of neck(neckline) to shoulder till elbow on my left hand alone.i can sense that this is an internal pain causing either within muscles or tendons. The nature of the pain makes me feel like it is not a pain caused to external injury(did'nt have any external injury) but possibly could have caused due to bad sleeping posture on a bare wooden cot or strained muscles during sleep. But the issue is the pain has not healed since it started 2-3 days back and can sense that the intensity of pain has increased today than it was the last 2 days.So far have taken 2 400mg Ibuprofen tablet one yesterday and one today; but no effect so far.No amount of stretching or relaxing of muscles seem to reduce or eliminate the pain.