It seems that you might be having some problem in your cervical spines giving you some pressure or irritation on nerves giving rise to pain, stiffness
on shoulder and neck.
There might be possibility of having extra cervical rib
Doctor might told you about x-ray report that loss of normal cervical curvature.
Consult orthopedic surgeon and get examined.
Meanwhile take some muscle relaxant
, short way diathermy and cervical traction will help a lot.
Ok and take care.