What causes shoulder pain?

I've been having pain in my shoulder for a few months. It started to come on slightly, but now its to the point where its unbearable at times. I've gone to the doctors and he said its in the scapula area. It feels like muscle pain, not bone pain. It hurts through to my chest too. I haven't injured it, or had a car accident or anything like that. Over time it just started hurting. I don't slouch when I walk or stand either, so posture isn't the answer. I've been scheduled for x-rays in the next week, but I figured I'd ask on here and see what people told me. Thanks