On Atkins you probably had low glycogen levels. The carbs probably filled your glycogen 'fuel tank'. The carbs you store also require about four times their weight in water. That could account for about 3# if you went from completely empty to completely full of glycogen, just as a motor vehicle weighs more full of gas. And as Nawchem said, high insulin levels are associated with water retention. So if your calorie intake was reasonable, it should come off pretty easily.