Clearly not 'bating enough with that arm.
I had limited range of motion with my right shoulder for about a year. I first noticed it at the gym, during shoulder exercises, as a dull pain that compromised my range of motion. I tried resting it the best I could but it never got better and over 7 months later, I could barely put my coat on.
That's when I went to my GP who referred me to a Sports Clinic doctor. He diagnosed me with Frozen Shoulder, which baffled him because he had only seen the condition among diabetics and menopausal women. He gave me a cortisone shot and showed me a bunch of exercises that honestly hurt like ****ing hell. But I did them 2x day and the improvement, while gradual, began almost immediately. I'd go see him for a fresh batch of exercises every two weeks --- and eight weeks later I was at 90%. Then my left shoulder started to "freeze", but because I had seen that movie before, my doc just gave me the same exercises and it was fine 3-4 weeks later even though I was much lazier about doing the exercises the second time around.