Weird. This exact same thing happened to me this morning. I think it was a long-delayed beat, followed by a THUMP, followed by a bit of faster-than-normal beating. I was brushing my son's hair. I stopped cold and went over to a chair and sat down. Didn't feel right for a little while. I quickly strapped on my HR monitor and my pulse was back to normal almost immediately: 55 or so.
I haven't been sleeping as well--been waking very early (4:30-5:30)--for the past two weeks. In the past, I've noticed this very occasional arythmia when I'm having sleep problems, or, alternately, when I've had too much coffee.
It's never happened while I'm running or racing, so I haven't been worried. And it rarely happens regardless. But yes: that THUMP will get your attention!