It does not matter if it is a new injury or aggravating an old injury. It may be an over-use strain or an actual injury. Treat the symptoms, not the cause. Swinging that hammer strains a large area from C-1 down to T-2 or even lower. I dislocated a rib swinging a splitting maul.

A physical therapist or chiropractor should be able to help you. In the mean time, focus on good upper back and neck posture when you relax or sleep.