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i have experianced this once myself
i am a healthy 21 year old, i experianced it after i had done some exercise i also felt very clamy and sick i broke my fast because it scared me
i have read alot of people mentioning heart palpatations etc during fasting, im not sure what causes it but it seems to be common and probably a sign to take it easy, ofc health always has to come first and if you were feeling too ill you would be best to have some juice or fruit but i dont believe its a sign of any heart problems or anything to worry about you could always get a doctor or something to supervise your fast if your wanting to go for longer they could check you over and make sure everythings fine just to put your mind at rest