The weight you gained is from food still in your digestive tract and water. It is scientifically impossible to gain 2.5 lbs in actual body composition weight over night. I can easily gain 4lbs after a meal. Eat a ton of meat/dense foods and drink about a half gallon of water and you will easily have gained a few lbs just from the material sitting in your stomach. Weigh yourself before a huge....well...You know. And then weigh after. lol Hence the saying, "I feel five lbs lighter after that du*%!"