Hi Bob, just wanted to note, being a hyper-cautious worry-wart (o:, keep an eye out & be careful even if the water thing makes sense. Because that's a pretty extreme reaction & sounds like more than water even if dehydration contributes, and nothing to mess around with -- I guess you know that from what you said though.
Might help to keep a very specific food diary from a day before the flush & throughout it, then you would have records & could compare. And what the other people were saying makes sense to me as to possibilities, I don't know much about flushing & side-effects.