Shoulder pain from swinging bat

Last April I hurt something in my left shoulder while swinging a baseball bat (I swing right handed). I didn't think it was a big deal and assumed it would go away with rest but here I am several months later and I still feel it. At first all movement hurt but now it has gotten to where it only hurts when I'm working out or when I sleep on it. It feels like it's in the anterior part of my shoulder. Any idea what this is and what kind of shoulder workouts I can do without aggravating it?