There is a whole bunch of different experiences a person can have on a WF, esp. in the first days before switching over to fat and disposable tissue-burning. So maybe not everyone has this one experience, but, yes. It's "normal".
(nothing to be concerned about)imv.
I have a theory, that as we become more familiar with our own bodies, and how they respond during a water fast, and when not fasting, we will understand more completely the "why" of it all.