I would highly suggest that you get in to see your rheumatologist
or your primary care doctor
about this issue. Any persistent or new pain in the bones should be investigated to make sure it is not a serious medical problem.
In your case, of course ruling out a flare of juvenile arthritis
is important. The likelihood of this will depend somewhat on what your experience with the arthritis has been so far. Your doctor will examine the shoulder and may also want to get some basic blood work to look for inflammation or infection.
In additional to juvenile arthritis, there are the more common causes of shoulder pain in young people. In particular, strains or sprains are quite common, and they typically occur either with repetitive motions, heavy lifting, or sports injuires. Therefore, make sure to tell your doctor when you go to see them exactly where the pain is and how it started (for example, falls or sports activities are very important to mention). Based on the information you give your doctor and their examination of your shoulder, they should be able to narrow down the potential causes of your discomfort and give you some recommendations about treatment.