Heart palpitation is very common while fasting due to increased need for energy(glucose) for your muscles and brain. basically that shouldn't scare you and will be resolved by itself(although you should rest). this usually happens to people with very stressed lifesyle. if you find that juices are helpful to relieve it don't drink too much juice because it slows down the process of fasting and will lengthen the fast. Therefore I recommend to avoid juices as much as you can.
I also recommend to check your blood pressure when feel your heart palpitation gets stronger, if you find your blood pressure only with minor changes compared to before it started (while fasting),than you really don't need to worry and may continue as planned.