Heart palpitations can be an alarming experience for those on a water-only-fast, but are in fact quite common and remedial.
Not only in my own experience, but that of thousands of others this is a common occurrence, where the heart is readjusting its electrical activity and responding to the release of toxins within the bloodstream. It is also a marker that the heart is in the process of strengthening itself, which it invariably will during the fasting process.
I am wondering how you are conducting your fast? because rest is vital at this time, and where physical activity/emotional involvement/anxiety, will only exacerbate this.
I would strongly advise you to listen to your body and submit to its wants and needs.