Since this morning, my left shoulder/collarbone area has been just pounding all day. It hurts a lot when I move it either backwards or upwards, and a little bit moving forward. I know I didn't fall or anything like that, but I just can't figure what the heck it is. I don't think I tore a muscle or ripped it. It feels like someone is pushing a sharp knife into my shoulder. Please help, some advice.