I think you have developed Peri artheritis of the shoulder and artheritic changes in the shoulder joint.
in this case only physio therapy can work.it requires regular and daily exercise of the joint to prevent more stiffness
and restricted movement of the joint.
keep your joint moving otherwise in the future you wont be able to button your gown from behind.
You go for regular exercise,
shor way diathermy.
ok and bye,