Left shoulder Blade pain

I am a retail sales assistant which consists of unpacking heavy grocery items such as Cases of soda and beers and other heavy items that we stock in the store as well as serve on the checkouts and use computers. I have developed a burning sometimes stinging pain in my left shoulder blade. Sometimes the whole shoulder and left arm seem weak and last night the pain caused me to drop a case of soda. The pain seems to ease a bit when supported by a chairback or when lying flat,hurts most when am standing or sitting without support against the shoulder or when lying on side in bed. Any ideas?

My problems also started out with pain under my shoulder blade 4 yrs. ago. Just this year I was told I had an inpingement in my shoulder and that the pain in my back was reffered pain from my shoulder.I had surgey on my shoulder but my back pain is still there and they are thinking of doing a EMG to see if I have a pinched nerve.( I also have tingling down my arm and in my fingers.) But I wish they would have checked into my shoulder sooner because now I have a tear in the cart. of my shoulder that they can't fix and I have to live with the pain of that for the rest of my life. It would probley be a good idea to check it out. And tell your Dr. that you want your shoulder checked I was missed diagnosed for 3 yrs. Good luck.