I think you solved the mystery! No maple syrup and just lemon water will surely throw your system out of whack. I know there are water fasters out there (I'm not keen on that) but when I check the Water Fasting boards, most usually drop out after 2-3 days.
The syrup provides the necessary nutrients and Sugar to keep your blood balanced. This is one thing I love about the Master-Cleanse -- the constant balance. Unlike eating a piece of bread (or starch) where my Sugar level goes up then crashes. Gosh, I hate that!
Glad you found some syrup and are starting to feel better.