Welcome to our family. If you read through the forums here, you will find that many of our members have suffered from costochondritis. Since auto-immune diseases (such as Lupus and MCTD) are diseases that cause inflammation throughout the body, we often get inflammation of the rib or the cartilage
connecting a rib. It is, actually, a common cause of chest pain.
MCTD is also quite common amongst our members here and it causes overlapping features of primarily three connective tissue diseases — lupus, scleroderma and polymyositis. Usually, with MCTD, one of these diseases becomes prominent and then a diagnosis can be made. You mentioned that your doctor was leaning towards polymyositis. It can, however, take months to years before any one disease takes prominence and many patients are left with a MCTD diagnosis for lengthy periods of time.
I am glad that you found us and that you decided to join us. As you've seen, our members are genuine in their care for one another and we are diligent in our efforts to provide information, answers, and comfort. Once again....welcome
Peace and Blessings
Look For The Good and Praise It!