Do you think your work is physically demanding or do you spend lots of time in front of a PC? Where exactly in your shoulder is your stiffness? between shoulder blades? over them? or more focused in the shoulder joint itself? or combination of the 3?

To have a better understanding of what it's going on, a proper clinical examination needs to be carried out. Irritation/inflamation of the capsule surrounding the zygoapophyseal joints (the little joints which articulate your vertebrae) can sustain a previous muscular strain and/or affect muscles such the cervical erector spinae, levator scapule and rhomboids if the segments involved are C3-C4 for the former and C5 for the latter mainly(via the dorsal scapular nerve). This is obviously just an hypothesis and as already said further investigations are needed. Osteopathic treatment, in your case, could be very effective and possibly resolutive.

Wish you all the best

Emanuele