Did the onset of pain happen shortly after a flu shot in that shoulder?
I had very limited use of my left arm - moving it more than 6 inches or so forward or back caused excruciating pain. I’d wake up several times a night from the pain. I found something called the rotatoreliever online. It was $150. I made my own homemade version with an ace bandage. The first night wearing it, I had no pain. That gave me confidence that the device would work. I ordered it. It came with a money-back guarantee that shoulder pain would disappear in 45 days. It took me 60 days, but I’ve been pain free and have had full range of motion for 5 years. I’ve since lent the device to a couple of other people, and it’s helped them as well.
Good question. An injection given too high up on the arm can cause a lot of pain (shoulder injury related to vaccine administration - SIRVA). The person giving the injection and the patient should be at the same level - both sitting or both standing. It also is more likely to occur when the recipient pulls down their collar/neckline to expose their arm.