This sounds a lot like a shoulder compression such as Amos suggested. It is why I was operated on earlier this year, (my spur of bone I mean,not yours). Mine was only diagnosed after two years of cortisone shots and all sorts of other malarky tried by doctors, by a physiotherapist and confirmed by x-rays. One small op and as good as new now.

Get thee back to thy physician and demand a physio referral toot sweet.