Hi Chris - Welcome back (possibly we've not met before as I've only been her just over 6.5yrs!)
Whom should you see? Suggest a specialist, as you wil need a 'proper' check on the heart, a GP cannot/does not have the equipemnt to so do. My heart problems were only discovered by a *top notch* hospital afore going in for a proposed (& further) shoulder op procedure. Myocardial infarction. Know exactly 'when' this occured as well - painful.
However. have 'also' costo problems and rib problems and sternum problems (and have had a vile bout of pleurisy in the past - left me with scarring) Have palpitations on a fairly regular basis and have high blood pressure - that I keep in check with alternatives (works).
Please get your problem checked out. OK?
As for the middle of the back? Thoracic. Get a MRI Saggital STIR done This will show up inflammation in the thoracic area and 'any' oedema/anything else going on. MUST be MRI Saggital STIR - OR a Nuclear scan. This last means rad expositure. A MRI Saggital STIR = NO rad exposure - and to be preferred!
My own intercostal diaphragmatic expansion had gone from 5.25" down to 2.5" - at least I got some expansion left, 'smile' (Used to do voice training...easily run out of breath now!

)
Hope this helps - but please do ge checked out - OK?
Molly C (France)
Keeping on Keeping on