I think it's to do with your sugar level /blood pressure because if it's low than you'll start to feel dizzy.
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