What Causes A Throbbing Pain In My Shoulder Along With A Knot?

I have been having pain in my left shoulder/arm for 3 wks now. It is even painful for me to raise my arm to take off my shirt. I also just noticed a knot under my clavical on the left side. It showed up last week. It is extremely sensitive and painful when touched. I had an xray that came back fine. I feel like the pain originates in the area of the knot. It is constantly throbbing in this area. I have also been more tired than usual in the past couple of months, but do not know it this is related.