Understand that on big guys (me included) 20 lbs on this diet aint no big deal.
The science is this: for every one gram of carbs in the body you are holding 3 grams of water. You ditch the carbs, depleted both muscle and liver glycogen, and thats a SHIT LOAD of water gone in a very short time. That weight WILL come right back on over the course of the days that you are eating high CHO foods, and bigger you are, the bigger the swing.