There's no way, physiologically or otherwise, that you can gain 6 pounds of true fat after one or two so-called "cheat days." 6 pounds of fat gain would have required you to consume and store 21,000 excess calories as body fat. It cannot happen in a couple of days.
What is happening is water gain due to consumption of too many starches. Eating too many carbohydrates without enough physical activity will cause bloat. Foods with a high carbohydrate level cause the body to store three times as much water compared to protein. This storage is in the form of glycogen, which requires a considerable amount of water.
The end result of eating too many carbs without enough activity is water weight gain, at least in susceptible people. In carb-sensitive people, carbohydrate loading (a.k.a. "cheating") causes fluid retention that results in a lot of temporary water weight gain. Recall that the liver utilizes water as it creates and stores glycogen from all the carbs one eats. Thus, water weight gain will show up on the scale as a heavier weight.
I do not have cheat foods or cheat days. If I want a slice of pizza or a cheeseburger or taco, I eat it. Since I have no forbidden foods, I have minimal desire to go off the rails. "Cheat days" are a part of the dieter mentality. I sucked at diets; therefore, I am not on a diet. As long as that burger can be worked into my daily calorie budget, it is not a cheat food for me.