If you already have a known arrhythmia such as atrial fibrillation or atrial flutter, it is quite possible that fasting led to some dehydration, and consequently a rise in your heart rate which exacerbated your underlying condition. Any state that causes an elevation in the heart rate (stress, dehydration, infection, inflammation, etc) can exacerbate/worsen someone with a known arrhythmia. Try to stay hydrated and avoid periods of dehydration as such. If you continue to have symptoms or feel that your palpitations are worse and/or associated with new symptoms, you should see a physician for further evaluation.
Thanks very interesting re dehydration.
Peter