I had the same problem some months ago. I thought I had RSI as the pain was so bad. I went to my GP who told me it was a trapped nerve. He carried out a very simple procedure and gave my neck and shoulder a quick crunch, it was completely painless and within a couple of days I was pain free. GP did say if his treatment did not work I may need a chiropractor's help but that was not the case. I have had no problems since. I would recommend talking to your GP first, it could be that easy to put things right.