Could a defibrillator cause my heart to race?

Rapid heart beats have many causes, so I wouldn't worry too much at this point. Have they checked your device recently? Is it due to be checked soon? If you do not have an appointment soon with your cardiologist, I would call the office and tell them about your symptoms. If they are concerned, they can easily schedule an appointment to check your device to see what your heart is doing when you feel it racing. It may not be a problem, but I would let them know just to be sure.