This is to a certain degree normal. When you're on a water fast, you get your energy completely from fat, i.e. your own body fat. Moreover, this fat is not of good quality, as all toxics that your body can't process are stored in it.
As long as your heart is beating regularly and, as you say, not really fast, then normally you shouldn't worry. Fasting is hard work for the body. Even though your digestion is at rest, other organs have to work very hard to deal with all the toxins that are entering your blood stream. It's the same like when you have a cold or indigestion for example, your heart will also be beating faster then.
With an electrolyte imbalance, the heart usually beats irregularly and there will also be other symptoms such as weakness, blood pressure changes, muscle spasms, etc. Normally, this doesn't happen from one moment to another either, you feel it coming. Also, the cause usually is dehydration rather than fasting, although both together can make a bad combination. I once had problems with this when during a longer Water Fast I tried to walk 25 miles in very hot weather. It was not like I was going to drop dead all of a sudden, but I can imagine that, if you don't listen to the signals that your body is giving you in a case like this, it could be dangerous.