Sitting Heart Rate Increases priodicly.

Hello My heart rate has been in increasing tendency since few months. I feel one full day every 10-15 days that my sitting heart rate fluctuates 100-130 while checking on oximeter during the day time of around 6-7 hours while working on my desk. At this time if I walk then it goes above 140-150 and I start little shaking.I have gone to ER twice in a year. Done ECG which is also normal. 2D Echo ultrasound are also normal. Have been diagnosed with gallbladder stone of 5.8 with my last er visit as they done all tests. My heart rate during sleep 45-60. Since few days, My heart rate whole day stays 100-120 during the day time and it decreases gradually moving to night. Doctor suggested nothing to worry but only I can understand when this happens. I don't know how this can be cured now. I was covid patient last year and am also a sportsman though now having sedentary lifestyle. Also, I feel shortness of breath while being in the theatre and while driving car since many years. pls help