It HAS to be water, you didnt eat 25,000 calories yesterday, so it must be water, constipation (not usually more than 1 lb), or scale fluctuations.
And to be fair, SOME of the 6 lbs from December 29th was probably water too. 6 lbs in 11 days isnt all fat loss. The real number is somewhere inbetween.
I confess to being a dedicated frequent weigher. If you do weigh more often than once a week you have to learn to acknowledge and accept the natural fluctuations. I fluctuate a lot which is one of the reasons I DO weigh often. I would hit a "low" one week and a "high" the next and think I had gained when in actual average weight I had lost. I just look at the lows and ignore the rest.