Had any injuries to that shoulder, or fall on it/hurt it recently??

Sounds like youmight havesome ligament damage, or one of your labrums is torn... Id goto a doc right away, dont risk having to use a crossbow for the rest of your life.

I had to go through a season once with a torn rotator cuff, and it wasn't fun. It does help if you keep your elbow down and next to your body when drawing the bow. It is awkward and uncomfortable, but at least you can shoot. It is worth a try if nothing else.