I have pain down the side of my neck and jaw, but that is from clenching my jaw under stress. I play for a while, and I guarantee I'll feel it.

The shoulder thing seesm to be related to the amount of time I spend at a computer regardless of whether I'm playing or not. So mouse height/location may be the key.

I have very limited range of motion in my neck due to wrestling in high school, but the shoulder pain isn't near the neck. It's down below the right shoulder blade, and will hurt for hours once it starts up.