The average adult human body is 65% water.
Most of the loss of weight during Water Fast is loss of water cause water is majority of your weight.
Even our fat tissues are almost 50% water.
How much water our body (cells) holds in depends on many factors, moon cycle being one of them, menstrual cycle and hormones playing also an important role in women and amount of water we are able to drink.
As the full moon is approaching (December 13th 2016), your body will tend to hold on more water (most of it during the full moon), and it will of course affect your weight.
You should not be concerned about your weight fluctuations during your water fast, it is all natural, and even if you are gaining weight, it is just water, it is impossible to gain weight during the water fast, other than gaining water and storing more water in your fat cells.