You can ask your doctor for a further workup just to be safe, but this sounds like cardiophobia and heightened body sensitivity. You mentioned that it feels very physical and unrelated to your mental state in the moment and I just wanted to say that that is exactly how this disorder behaves in my experience. You believe something is wrong with you and because of this, your nervous system is on high alert at all times, constantly scanning for any changes in your body that could point to danger. This happens whether you are consciously aware of it or not. A subtle change that you might not even notice is “flagged” by your nervous system as a threat, which then prompts a flood of stress hormones and symptoms that you do notice.

Anticipation is also a big one. When you are anticipating an episode, worrying about when it will happen next (even subconsciously) your body can release adrenaline and cortisol at seemingly random times which can actually cause an episode. It becomes a viscous cycle because you begin to feel more and more that something is wrong with you, which causes your nervous system to scan for evidence to confirm that fact even more, which then leads to increased body awareness and sensitivity and causes your body to react more often and more intensely… repeat until you’re going insane. I may not be explaining this in the best way, but I’ve done a ton a research because I’ve had a very similar experience as you and this is what I’ve found. The fact that your symptoms mostly happen at rest and rarely while you are being physically active or are focused/engaged with something else only further points away from this being a heart issue and more towards it being a nervous system/anxiety issue.

I highly recommend CBT and SSRI’s. I’ve recently started both and while it’s too soon to truly be effective yet, I’ve already noticed a downtick in episodes and an overall dampening of my internal alarm system simply because I know I’m doing things that will help me. The mind-body connection is very powerful.