Muscle Problems in shoulder?

I'm 14, almost 15, and I play Baseball. I throw about every other day, but not that much. Starting about a month ago, my trapezius in my right shoulder has been doing really weird things. I just recently started growing at a really fast rate, so i dont know if that has anything to do with it. But every once in a while, when i move my right arm or lift something, it feels like the muscle is tearing one fiber at a time. It isn't excruciating pain, but it hurts quite a bit. But it only hurts when it "tears". I dont know if it is tearing, but thats what it feels like. I make sure that i stretch before throwing, swinging, working out, etc. But i make sure I don't stretch too much. It happens kind of randomly, but always when i move something that requires the trapezius. Thanks