Can't believe I may have had a cardiac episode

Firstly, I am, or rather was, a mechanic, not a cardiologist, so please excuse my few words of possible reassurance for until you get a cardiologist to see you.

When I read what you wrote I right away thought, Angina and indeed, looking up causes of ST depression, partially blocked arteries can cause this, and so your pain.

The thing I understand with Angina though, is that it tends to be worst when you are pushing your heart, then the restricted arteries cannot get enough blood to your heart, hence the pain, I presume though that as you are "very fit", you push your heart at times yet never felt this before?

In which case there is something known as Prinz Metals Angina or Variant Angina, what this is, is your Autonomic Nervous System over reacting to anxiety or stress, such as being on an aeroplane maybe?, so unlike Angina caused by a blockage that hurts when you push your heart, Prinzmetal angina tends to be more when you are at rest and stressed, I get it sometimes.

It is caused by vasospasm, a narrowing of the coronary arteries due to contraction of the smooth muscle tissue in the vessel walls. Just as your Autonomic Nervous System constricts the arteries in your legs when you stand up to stop gravity pulling all the blood away from your head, it can also constrict the arteries in your heart, therefore causing the same symptoms as a physical blockage from Coronary Artery Disease, but without there being anything at all wrong with your heart.

Heartburn or acid reflux can give you a pain like that or even mimic a heart attack.

Or after lugging luggage around the airport:

Costochondritis is an inflammation of the cartilage in the rib cage. The condition usually affects the cartilage where the upper ribs attach to the breastbone, or sternum, an area known as the costosternal joint or costosternal junction.

Chest pain caused by costochondritis can range from mild to severe. Mild cases may only cause your chest to feel tender to touch or some pain when you push on the area of your chest cartilage.

Were you given a d-dimer blood test? I was thinking maybe you could have a pulmonary embolism or emboli, particularly as you were on a long haul flight. Are you taking any hormonal medication?