Hello I just wanted to say I know how frightening it can be to feel your heart racing or palpitations Iv'e also had all the tests and everything came back ok except for the ectopic heart beats. Ive had blood test for heart failure, 48 hour ecg and a scan of my heart they said they couldn't see any reason for getting them and it wasn't dangerous. I sure feels like it at the time.
It was so bad one day that I walked into a doctors surgery and asked to be seen they phoned an ambulance for me because my heart beat was all over the place and my Resting heart rate was 120bpm Its so frightening by the time the ambulanced arrived it had gone back to normal they did an ecg in the back and that was fine too that's when they sent me for all the tests.
Things I do now when I can feel it coming on or happening is splash my face with cold water lots of times. Yawn a lot it apparently resets the vagus nerve along with humming, breath through a straw or a pen and bear down as if going to the toilet. Obviously I don't do it all at once ;
They have helped me massively. I tell my self as much as the panic sets in that My heart is perfectly fine and it will pass whilst trying one or two of the above.
I have also watched a youtube video from a hospital where a patient has gone in with tachycardia and they get him to blow through a syringe and then lay him down fast, raise his legs and you can see his the heart rate dropping down.
Sorry I have no advise about it happening whilst taking hrt, I suffered from the above before taking hrt.
That's in my toolbox for when all else fails

I think sometimes it's good to have some things we can try to help ourselves with I call it my little toolbox of tricks