I'm on day 26 of my 3rd Master-Cleanse and I had a similar experience, which I described to my wife as "heart flutters." This lasted about 3-4 days and, although I don't recall which days, it was early in the fast. It definitely freaked me out but I stuck with the fast and it did eventually stop happening. I believe this also happened to me on my second Master-Cleanse which lasted 20 days.
I certainly wouldn't encourage you to continue with the fast, though, if you are uneasy about your health - especially something like your heart.