im currently undergoing the same shoulder pain/rehab
I woke up with pain in my shoulder , just reaching to grab a cup beside table causes pain
Notice a pattern
Shoulder pain worse in the am after lying on that side better by the end of the day.
Diagnosis with impingment syndrome and had PT
Pain got worse , started to have difficulty just getting my overcoat on.
Quit sports for 4 weeks
Pain resolving with PT and massage .
It takes forever to wait for MRI in my country
During my off time I developed adhesive capsulitis
I suspect that I likely started with had a either a rotator cuff tear or posterior labial tear causing the pain.
What I found helps is
1) no tennis and just rest during painful episodes.
2) pain resolves but stiffness develops, so I do self stretching and specific RMT treatment for my shoulder.
3)Have not considered cortisone or surgery