I have never had any irregular heartbeat episodes. In fact I couldn't imagine what that condition would even feel like until I experienced it. I awoke feeling anxious, with a really odd sensation of 3 heartbeats, a pause, one heartbeat, etc. There was no "heart racing" effect, in fact it was the slow, erratic nature of the episode that really puzzled me. It was quite strange to experience, and I couldn't get comfortable in bed, fortunately, the effect was transient. After an hour or so the symptoms disappeared.
Normally, I have few problems with tolerating medications, vaccinations, and such. This was a very odd episode. Of course, correlation is not causation, but the occurrence was 48 hours after my second dose, and it was nothing that I had ever experienced before.