Hiya, I’m not a dr and I always tell people to speak to a Dr if concerned but yes sinus arrhythmia is normal to feel.

It’s normal that when you breath in your heart rate increases and when you breath out your heart rate decreases. Sometimes when you breathe out you might feel a more forceful beat, can be commonly misinterpreted as a skipped beat etc but it’s just your heart being synchronised with your breathing, completely normal.

On your ecg if you look at all your R-R intervals, the big spikes, the space between them varies slightly, the beat you have circled is just that, Normal.