I bet it's the same thing i have, impingement syndrome.
About 6 months ago i woke up with a sore shoulder. I thought maybe i just slept on it wrong. After about 5 months of the exact same symptoms you mentioned, i went to a chiropractor. Impingement syndrome is one of the most common causes of pain in the shoulder. It results from pressure on the rotator cuff from part of the shoulder blade (scapula) as the arm is lifted. I get adjusted twice a week along with an ultrasound treatment and muscle stimulator treatments. He told me right away to take 4 Aleve twice a day for about a week to take the inflamation down. That helped alot but my pain (although much less) came back right after i stopped the aleve. So, one month into treatments he has me starting to do exercises but it still bothers me. I'm ready for a cortizone shot which would be the next step. Hopefully if you have the same, it wont be as bad.
The pain is most severe if I raise my arm straight out to the side 90 degrees or try to reach behind me.
That sounds exactly like what i feel.
I have been scared as hell to pick up my bow and give it a try. I'm afraid it will hurt too much and if it continues, i may have to think about a Xbow. I'll do everything in my power before i have to resort to that.
If i were you i would get a chiropractors opinion too. Hope everything works out and let us know what you find out. If you never have had a shoulder injury in the past, it makes it harder to believe that it is a rotator cuff injury. More than likely, impingement.