[AnimalMuppet]

I have those. At least in my case, it's a premature ventricular contraction. What happens is that the nerve fires too early, and the heart muscle says: "What? LOL, no. I'm still on my coffee break." But blood keeps arriving at the heart, so the next beat is stronger because it has more blood to move. They can get strong enough to hurt a bit.

In my case, it seemed to be caused by Claritin. When I stopped taking Claritin, I quit having these. Years later, just recently, I've started having them again.

Disclaimer: I am not a doctor, and this is not medical advice. Your symptoms may be something else.