Many moons ago

I twisted a collar bone in the joint (clavical ?) which didn't hurt at all at the time, but swelled up and was very painful for years after. As part of this I now have over stressed chest muscle holding the shoulder in place. Something I'm not aware of until I relax and then the muscle relaxes. But when it does so its very sudden and feels like a palpitation. A big one. Coupled with an amount of pain that radiates down the left arm and covers the chest, well, you can guess what that might feel like.
Thing is panic sets in and heart rate goes through the roof.

or used to. Now I know what it is there no panic and I can control it.
Thing is, obviously, unless you've had a heart attack in the past and know what that's like, you do assume the worst. Something my GP couldn't possibly understand. Arrogant tw@!
Anyway. Not to be taken lightly, but there are looooads of symptoms in the upper left torso that can be very worrying.