Hi how long has it been since they checked your pacemaker and when was it put in? They are set for a certain heart rate and when you excercise too fast maybe the pacemaker doens't beat fast enough for you. Also, the pacemakers have to be changed about every 10 years. I would get the pacemaker checked to see if it is functioning properly. Maybe walking is best for you.
Mkh9
I think it's to do with your sugar level /blood pressure because if it's low than you'll start to feel dizzy.