My shoulder is killing me?? Please help..?

I'm the catcher for my softball team, the season is over... But ever since I started catching (this is my first year) my left shoulder (the shoulder I hold my catchers mitt up with) has been killing me. I can't even sleep on it. I'm only 14. I play fastpitch, so things are much more advanced, so the positions are much more crucial. Any idea on what I should do? I can't even hardly workout in the gym because of this. :/