Thank you for your post. This came up when I searched for fluorophenibut and heart. I was born with an extra electrical pathway on my heart (just recently discovered at the age of 32, it's called WPW and is associated with a type of tachycardia called supraventricular tachycardia) and sometimes on this substance I noticed a kind of pressure in my chest, around the heart region. I've used loads of stimulants but mostly benzos and opioids, I'm clean now thank god (not really god but you know...) but I'm still dabbling in some of these variants I've never tried before.
This morning I had my milligram scale put on oz instead and didn't really notice at first, I did thought it was more powder than I normally mix in with my water (it tasted horrible by the way

). But before I drank it I realized that it was almost 1.3 grams which is way more than I'm currently on daily, which is roughly 400-600 mg. So I saved it for later in the evening, it's maybe 2 hours ago now since I drank it and I was recently biking to the store to buy some groceries and noticed this pressure in the heart region. But while writing this the pain or whatever to call it, the "feeling" had subsided almost completely.
So it checks out with your timeline as well and also with the physical activity. And when you wrote how horrible the withdrawal was for your heart and that you got tachycardia (my heart abnormality is associated with a type of tachycardia called supraventricular tachycardia) I got quite the scare. I'm getting off this now. I only ordered a total of 20 grams and it should be the end of it soon. So I'm going to use the last powder to taper down, I'm going to use some whenever I feel 'weird'. I'm completely drug free otherwise so it'll be easy to feel when somethings off.