Robert > Higher protein consumption and hard exercise both require more
water.  Your protein consumption isn't terribly high though.  Heavy
carb consumption can also take a lot of water.  For example, when
reloading muscle glycogen, your body needs 3 grams of water for every
gram of carbs.  Maybe your carb consumption on the previous day (before
day one) had slightly dehydrated you?

Interesting points.  I am fairly sure that my dehydration was triggered
by my dramatic switch in diet.  The glycogen reloading idea has some
merit  -- perhaps my body in both sensing the oncoming dearth of carbs
and reeling from the high intensity workout yesterday stepped up the
glycogen reloading process.

And, no I don't weight 100 pounds.  After having gone nuts with the
carbs of late, I believe it's just the normal switch to ketosis that
kept my calories so low yesterday.  Break even for me, I believe, is in
the 2900 calories a day range.