You could also capture this via portable heart monitor.
It seems you have an arrhythmia during exercise (faster heart rate than expected), and probably have the same underlying cause as arrhythmia (faster than normal heart rate) at rest. That would indicate the arrhythmia is triggered by a fast heart rate.
For some insight, physical exercise is related to vagal modulation of HR during exercise independent of aging. Experimental data have shown that vagal activity prevents ventricular fibrillation during exercise and that exercise training confers anticipatory protection from sudden death by enhancing cardiovascular autonomic function... damage to the vagus nerve my decrease parasympathetic tone and allow increases in heart rate...cardiac vagal modulation dysfunction. There could be a hyperthyroidism condition.
If you are experiencing a faster than expected heart rate, you would want to eliminate a potential abnormaility that could put your heart rate into a dangerous range that could damage your heart muscle, etc.
Mayo Clinic: "There is also experimental and clinical evidence that augmented sympathetic outflow is related to arrhythmogenesis and cardiovascular morbidity and mortality (20, 21). Rapid loss of vagal activity followed by sympathetic activation during mild-to-moderate exercise intensity may be one potential mechanism for adverse clinical events, and the present data support the concept that good aerobic fitness may exert cardioprotective effects by enhancing the cardiac vagal function during exercise".
You may want to see a doctor, and if the problem is consistant with exercise, a stress test would be able to diagnose.
Thanks for sharing, take care.