Sleeping on a shoulder that already hurts
Pillow height matters more than mattress price, and the arm you hug with matters most of all.
Side sleepers wake with the lower shoulder folded under them for hours. A pillow tall enough to keep the head level with the spine takes much of the load off.
Hug a second pillow with the upper arm so it does not roll forward and drag the shoulder blade with it. This single change resolves more morning stiffness than any new mattress we have heard about.
If the pain wakes you rather than greeting you in the morning, that is worth a medical opinion rather than a better pillow.