A rib subluxation in the thoracic spine, stretching from the lower neck to the kidney area, can radiate along the rib to the breastbone area. Then you would have mid back pain, pain somewhere under the armpit and under the breast.
An important differential is: do movements of the back, chest, arms, breathing provoke the pain? Then it's probably musculo-skeletal. Obviously referred pain from the internal organs, kidney, pancreas, lung... can refer to these areas too.
If the joint between the rib and the breastbone becomes very tender, and even swollen, then you may be developing a Tietzes syndrome
... read more at this page.
Have you seen a chiropractor? This is a bread and butter problem for us. Find a good chiropractor
may be your next stop.
I hope this has contributed.
Dr Barrie Lewis