The type of shortness of breath you describe could be due to pain, anxiety, an abnormal heart rhythm or a lung disease, such as asthma. The pain of costochondritis is notoriously unpredictable. A bone scan is not a good test to establish the diagnosis. If the chest pain is also associated with tenderness of the sternum and/or the rib attachments, then the diagnosis of costochondritis is likely to be correct.
With a "strong family history of heart disease" your concern about the interatrial septum motion abnormality is warranted. You should definitely see both the cardiologist and the rheumatologist.
These muscles really ache by the end of the day, speaking makes it worse. I gave up work so do not use my voice anyone during the day but its not got any better.
I have to psychically force these muscles to work quite a lot as my breathing gets very shallow, bit like trying to lift an elephant with your little finger, soo tight muscles.
I sleep proped up against the wall now as as soon as I lie down I cannot breathe properly. Lying on either side hurts also and keeps me awake.
I did have a chest x-ray but that did not shed any light on the problem.