A knot under your shoulder blade is often a muscle or tendon that has gone into spasm. And yes, the inflammation from RA can affect your tendons, ligaments and muscles, as well as your joints. Also, using your body when your joints are in pain or have a limited range of motion can put stress on other systems in the body, causing tension which can lead to muscle/tendon spasms, etc.

Keep in mind that I am not a doctor, I don't know you or your medical history. Therefore, it's a good idea to see your doctor and get her/his opinion. If the pain gets worse, spreads or you develop other symptoms, go to an emergency room. I'm pretty sure that this is a muscle spasm, but I said, I'm not a doctor and it's better to be safe than sorry.

If it is indeed a knot of tension, painkillers help, as does a soak in nice warm water or putting a heating pad on the shoulder blade. While the pain is there, try to avoid lying on your left side. As well, it might help to reduce your activity level for a few days. If that is not possible, keep your shoulder and neck warm and do gentle stretches throughout the day to keep the area from tightening up again

Good luck!