I've had this phenomena too, and when I asked my PT about it he thought it was most likely the Dura Mater, the Dura protects the cord and run through your body. It is very sensitive and when inflamed it can transmit pain from top to toe, along its course.
Disc degeneration might lead to an inflammatory response affecting nerve roots, but also surrounding structures as the Dura Mater. When you lift your arm you stretch muscles and the dura mater as well. But maybe there are other explanations, the body is complicated, structures connect with each other.