I think you may have Rotator Cuff tendinitis, bursitis of the shoulder or Shoulder Impingement.
The rotator cuff is like a lip on the rim of your shoulder socket that prevents the ball from popping out.
As you can imagine, the range of movement performed by the shoulder needs a lot of muscles and tendons pulling and supporting it in all sorts of directions and this means there is a lot of scope for injury.
People who reach up a lot, such as basketball players, swimmers and tennis players and those who paint, lift or in the building trade are particularly susceptible.
This sort of situation could also lead to overuse and inflammation of the fluids of the bursa that lubricate between the tendons running across the bones.
Shoulder impingement results from the bone on the top of your shoulder (the acromion) pinching the tendons nearby as you lift your arm.
Ask your doctor or a physio for confirmation as we cannot diagnose but they are the 3 most likely explanations and a combination of Atrogel® applied externally 2-4 times a day and Atrosan®' internally to reduce the inflammation would be the best natural options.