I was taking 2000IU daily.
For more details, I'd usually take it late morning with breakfast. I'm a normal weight (155lbs), exercise, eat healthy, low stress, in my 30s, good blood pressure, don't drink coffee. I'd notice the heart issues usually in the evening. Sometimes they would occur almost daily for a week, and then they'd go away for a couple of weeks. Then I'd experience them again one day, and not for another few days. They would occur for about a few seconds or a minute when I was at rest and not doing anything active. It's hard to describe the feeling but it was kind of like a muscle spasm occurring in what I thought my was heart. Or, like some strange beating/squeezing sensation that would occur once, then again 5 seconds later, then a couple of seconds after, then that was it. Or it might return after a few minutes and happen again.
It was an intense and serious enough feeling that it would make me pause whatever I was doing. It would make me think that one of these times it would be a bit worse, I'd end up collapsing, and that would be the end. I'd usually do a bit of cardio for a few minutes because like I said it only seemed to happen when I was at a resting heart rate.
I should probably get my heart checked because there's a good chance the connection with the vitamin D supplements is just a coincidence. Nonetheless, I'll pay attention to see if there seems to be a relation between the two, because although it's unlikely, stranger things have happened.