Rheumatoid arthritis (RA) is an autoimmune disease in which the body’s immune system attacks itself. The heart can be involved, however, with inflammation of the valves, inflammation of the sac around the heart (pericarditis), or inflammation of the heart muscle itself. Without fully evaluating your findings, I cannot say for sure what is happening with your chest pain, and what the cause of it is. However, pericarditis can cause chest pain, so it would be appropriate to ensure that you don’t have this as the reason your chest hurts. That said, the most common cause of chest pain in children and teens is chest wall pain. It’s not necessarily reproducible with pushing on it, it is often sharp and hurts worse when you breathe in, and is not due to the heart in any way. It is benign and does not cause problems. Your GP should be able to make tell by the history, examination, and an ECG if this is the cause of your pain.
Reactive arthritis is not associated with cardiac involvement, however.
Sorry, I meant "diagnosed with reactive arthritis" - I'm going mad!