My heart rate elevates rapidly during exercise. Should I be concerned about it?

Hi doctor,

I am taking Metoprolol 25 mg and Losartan 100 mg for HBP and diastolic dysfunction. I have lost 50 lbs over the last three months by doing a cardio exercise (jogging, row, and weights) four times per week for one hour each session. During this time, the resting heart rate dropped from 70-75 to 43-50. GP recommended going off Metoprolol - resting heart rate back up to 70; jogging one minute at 4.5 miles per hour and heart rate is 146 bpm within 30 seconds and up to 165 bpm after the minute. When on Metoprolol, jogging 1 minute at 4.5 miles per hour and heart rate is 133 bpm in the first 30 seconds, 140 bpm after the minute. I am now back on Metoprolol. How low is too low for resting heart rate (given I am not a world class athlete that runs marathons)? And is there a condition that causes the heart rate to elevate rapidly during exercise that I should be concerned about? No symptoms with a low resting heart rate and have no lightheadedness. I had a cardiac cath test three years back, and I am currently taking Metoprolol 25 mg, Losartan 100 mg, Omeprazole 40 mg, Flonase, and Zyrtec.