Not a doctor. You could have crushed your vertebrae cartilage causing a nerve to get hit because of the stenosis from bouncing the weight up and down with shrugs. Know a few that have done this with shrugs. Especially those that take a massive weight to get an inch range of motion. It becomes a great tool to crush nerves. Mike Mentzer suffered badly from trap pain. Hopefully the nerve will settle in time. If not I bet a doctor will order a MRI to see if the vertebrae have normal separation between the cushion cartilage. If the nerve settles down use a lighter weight. A full range of motion with no bounce. Do them like a robot. You might have to drop shrugs off the list of exercises you do. Another option is dumbbells shrugs. They work with less weight and less crushing force.