What Causes Pain Under The Breast?
can be costo-chondritis.
Detailed Answer:
Hi.
Thanks for your query and elucidate history.
IT is good that the CT scan and Cardiac check-up are normal and hence ruled-out.
The most probable reason I can think of with your history and findings you noted is:
Costo-chondritis along with the muscle swelling you suspected correctly. There is a local pain, swelling, redness and tenderness.
The back-pain can be a ''referred'' one.
The simple treatment of anti-inflammatory medicines like Ibuprofen or Diclofenac, (whichever suits you) in a proper 8 hourly doses for about 5 days should solve your problem. You may apply local rubrafacient (increases local warmth) ointment, rest to the part ( use the upper limb least possible), avoid any movement or work which gives strain on this part will help to solve your problem.
Please observe there is no local rash-like line on the affected side as one gets on herpes ( easily missed).
I hope this helps you and feel free to ask if you have further queries.
Wish you an early recovery.