While gaining 20 lbs of tissue in two weeks seems implausible, carrying around 20-24 lbs of excess water weight also seems implausible - unless one is extraordinarily obese which you clearly are not.
That's just it. I don't believe for me the 20-24 pounds of water (actually 16 pounds of it thru the first 14 days as explained below) is excess water... just "normal" water that a fasting body will deem superfluous during a fast. Gaining it back as fast as i do while avoiding any and all sodium that is not contained naturally in foods is what i base this thinking upon.
I wonder what your definition of "eating lightly" is - considering that that after an extended fast, one's metabolism will be dramatically slowed down for quiet some time. This makes it very easy to gain weight. What would count as a calorie restricted diet normally might be a binge to someone who just fasted for several weeks or more.
When i say eating lightly, i am talking about a vegetable and fruit centered diet. To be sure my metabolism slowed down in the fast, perhaps by about 1/3... perhaps going from about 2700 to about 1800, so perhaps my average metabolism over those 14 days of refeeding was 2000. To gain 20 pounds of fat in 14 days, it would have been necessary to eat about 7000 calories per day with a 90% vegetable and fruit diet when my average was much closer to 2500 :). Having been engaged in first stage resistance training from day 7, I estimate have gained about a pound of muscle and a pound of fat leaving 16 pounds of what i believe could only have been water (along with maybe 2 pounds of intestinal content). What this 16 pounds of water looks like on my body is muscles that have been filled with it, making them more pronounced while the body fat % has remained the same.