I had something like what Tony mentioned. Something in the shoulder was out of alignment. Low grade ache most of the time, pain when aggravated.

It fixed itself during a minor fender bender, where I was tossed forward against the seat belt. Said shoulder belt being over the good shoulder. I speculate that the jolt stretched everything out and let it snap back to its proper location. Instantly I could tell it was better.

I don't recommend the specific method, one was hurt but the car was a write off.