I've noticed this a few times when I have increased cardio in attempt to 'get fitter'. I start of gradually increasing intensity and duration of cardio (cycling or, more recently eliptical trainer) and noticed (usually the day after) that my heart rate is not 'even' I guess the term is arrythmia?not worrying and as I continue (avoiding over doing it) I get to a level where it stops occuring, almost as if the heart is now 'happy' with my level of fitness.
Then after a while of no CV work (injury or downright laziness) the same happens as my fitness decreases. I did get an ECG one time because of it but, of course, nothing showed up.
Not particularly worried by it just wondered if anyone else noticed this.
I know the conclusion should be Get Fit & Stay Fit!
BTW: 51yr old Male Powerlifter type training, 260lb (reducing through diet & exercise) meds: BP meds Ramipril & Bendroflumethiazide.