What do you advise for heart feels like it stopped for a sec, then beats hard once?

Palpitations see Doc: If you have other symptoms like sweating, shortness of breath, dizziness or chest pain with the skipped heart beats, you should seek immediate attention. Otherwise, see your primary doctor. Palpitations are often benign and typically we don't feel our heart beating at all. The hard beat occurs after the pause as the heart has more time to fill and squeezes harder to empty the extra blood.

Premature beats: You're describing premature beats: the heart beats early but you can't feel that beat because it's only partly filled. Then it pauses a moment to resume its former rhythm. While it's waiting (only a few hundred milliseconds) it has more time to fill, so the next beat is extra forceful. The medical term for the forceful beat is "post-extrasystolic potentiation". It's normal. Fear not.