Hi ! From your history, it seems that your upper respiratory tract
infection has spread to the lower respiratory tract, causing possible pneumonia/fluid in the pleural cavity
which is causing pain and difficult breathing. Costochondritis
will not cause so much of breathlessness as you have described. I would suggest to get yourself reviewed by your treating doctor who may examine your chest and heart, and advise you for a plain X ray
chest/ECG to exclude this.
Wishing you an early recovery.