It sounds like you my be under some tension for some reason.  OR . . . you should get checked for heart/circulatory problems.  Heart attack symptoms sometimes show up in the shoulders early.

Len

You might also try looking for some weight training exercises specific to the shoulders and upper back. Try to strengthen those muscles so they are more resistant to the tension.  I do lat pulldowns and standing row on a machine.  Standing rows could be done with free weights but not lat pulldowns.  Shoulder shrugs with or without weights may also help.

Len