Since I've been reviewing this board, seems like a lot of you have similar shoulder related injuries and can't seem to figure out what the problem is. I had the same type of problem and eventually got it diagnosed and figured out.
I had never heard of Shoulder Impingment Syndrome until my doc told me about it.....he's a world leader in shoulders so he happens to know a thing or two.
Read the following for a good explanation:
http://emedicine.medscape.com/article/92974-overview
Hope this helps!