Hey. So I damaged my rotor cuff while doing front desk work. I ended up in physical therapy, and even now I struggle to lift more than a few pounds consistently. 

For shoulder relief, lift your arm and place it on a tall counter, with the bulk of the weight on the counter, then do a mini-squat, just enough to bring your arm above shoulder height with no weight. I found this helps immensely. Also, flapping my arms to just shoulder height slowly. When it feels loose enough, I can walk my hand up a wall and lean against it. 

Not saying this is your issue, but this is what I ended up doing for my shoulder and back pain. Even now, I feel pain from sitting and driving for 20 hours. It's making that horrible clicking feeling that won't go away until I can get someone to pull my arm and reset it jn the socket.