Low pulse when active, normal when resting

I am experiencing dizziness and fatigue. I take blood pressure medicine. I just started taking Aceno and my bp is staying pretty normal. When I am doing strenuos exercise, I feel terrible, weak, sometimes dizzy, and my pulse is 46 to 55. When I am resting it is 80-100. What can cause this?

[zimm]

I am also on high BP midicine that keeps my heart rate low during workouts.  I experience the same symptoms as you.  
Are you using the same monitor while taking the resting rate?

I also have A-fib that I always attribute to the drugs the doctors prescribed.

** the heart monitors on the machines at the gym do not read my heart rate proberly and often can not read it at all.  Also when they do read the heart rate it is very low --- 50-60 bpm.
It really isn't the monitors that are not working properly, they just can't read my irregular heart rythum.
I do have days when the monitors read my heart rate as 'normal'.

Just throwing this possibility at you, perhaps you have atrial fibrillation that screws up these cheap monitors.  If so it is important you get on a blood thinner if you have a-fib.

Good luck