It could be too much poundage, too much shooting or a combination of both. I had the same thing happen and got tendonitis in my shoulder. So I went from 82 lbs to 64 lbs, gave my shoulder a rest, and it healed.

when I first started shooting it did that to me 2. I'm not sure why and may never b sure why. It doesn't bother me anymore but man, for those first 3-4 weeks that I first picked up a bow it bothered me. I think it had to do with getting the shoulder in shape. If you've been shooting for a long time i'd have to agree with the rotator cuff probs.

like others has said could be burisitis, could be a rotor cuff tear the only way to find out is to visit your doctor, hope it aint to bad yet