weight lifting and heart rate

Is it normal for your heart rate to drop a lot after lifting small weights?

I'm on 25mg of atenolol for fast heart rate due to an adrenal problem. Was
told after running on a treadmill stress test that I should exercise while
waiting for my endo appt.

If I walk fast on the treadmill, my heart rate is around 110, then slows
down to about 85 awhile after the treadmill.

But if I lift small weights (15lb) for a few minutes, my heart rate plummets
to 55 for several minutes. Is that strange?

I had an ultrasound of carotid artery, an artery in my stomach, etc. all

Thanks again.