I Have A St. Jude Medical Pace Maker Which Was

I have a St. Jude Medical pace maker which was implanted due to very low heart rate. Very light exertion causes me severe shortness of breath. I discovered that although the pacemaker is calibrated at 70 and 120 pulses per minute, my heart rate never goes above the lower limit of 70 which does not make sense to me. Any ideas?