Shoulder pain while running?

I'm a competitive swimmer and I have a history of tendinitis and scapular dyskenisis. I'm a really bad runner to begin with and we have to run for dryland in the morning. So, whenever I run, I have really intense shoulder pain in my left shoulder all through the front and along the back. I've tried loosening up while running and increasing my strides but nothing helps. Any suggestions or ideas of whats wrong? I would really appreciate the help! :)