Hi Gracecat, so sorry you have had this frightening experience & yes I've found Utrogestan intolerable for various reasons, the worst being that it badly upsets my heart rate - making it race uncomfortably - & rhythm, the ectopics I'd been plagued with in peri (before HRT) being confirmed on ECG to have returned on Utro several years postmenopause.
As for timing of improvement on withdrawal from Utro: my heart rate decreased & rhythm stabilised within 48 hrs, but I had simply swapped back to another HRT product & not gone cold turkey. I do find that changes in oestrogen dose can also upset heart rate & bring back ectopics for a couple of weeks, as well as thyroid meds changes. I think if we are especially prone to palpitations, as you seem to be if you had them some years ago, we are perhaps unfortunately overly sensitive to changes in hormone levels.
Stress of course makes matters worse & I can absolutely relate to how difficult it is not to stress about them, causing a vicious circle.
I'm not sure from your posts whether you are still on a little oestrogen, or whether you've perhaps reduced the dose, but if you have felt obliged to reduce or stop all HRT - entirely understandably - it could be that your heart is also now reacting to the drop in oestrogen & may take a little while yet to calm down.
My feeling is that your cardiologist will very likely find nothing of concern but do keep the appointment so they can check you out thoroughly & put your mind properly at rest. I had the whole range of cardiac tests in peri, with nothing other than benign ectopics found.
If at any time you feel things are really not right & you need to get checked again as a matter of urgency, I would either ring your GP - ours can do a simple ECG in the surgery - or go back to A&E.
Meanwhile if you can do some controlled breathing exercises these can really help to calm the body. I found I needed to do them for around 20 mins for the relaxation response to kick in & they may need repeating as often as you can manage it over the coming weeks, if you feel you want to ride out the cold turkey of stopping HRT. Anything else that helps you relax & distracts you from the natural anxiety at this time will be beneficial.
I hope this helps you feel less alone with it & that you will soon be feeling much better

Wx