This could be serious... Not trying to freak you out but my problems started out this way...
Basically I had/have a protracted shoulder girdle or rounded shoulders... On top of this it has either causes or been caused by a winged scapular.
Imagine dominoes falling down in a big line... One thing causes another and the problems mount up and eventually you get really damn hurt.
The causes can be many, bad posture - do you have an IT based job? Tight chest muscles, tight or weak scap, weak rhomboids, weak rear delts and weak lats.
So what I've been trying to do is get all of those weak areas stronger, focusing also on flexibility and mobility exercises as well. I've no idea how long it'll take to fix the problem, even if it will but either way strengthening those weak points is probably a good thing.