Left lower shoulder pain

I have been experiencing lower shoulder pain for the last 2 months.
I had been doing some heavy lifting about a week prior to this all starting.
I have been told that I might have a rib out of place. Could this be my problem? It went away about 2 weeks or so ago and then I lifted a few heavy boxes the other day and the pain came back again.
When I lay on my stomach and try to lift my left arm up it really hurts.
The pain is always there when it does flare up. A heating pad makes it hurt worse. Can you give me some advise as to what this might be?