Hrm.

Did you weigh in the morning and then again at night?

We all gain 4-10 pounds each and every single day. It's from consuming water and food. (They are not weightless) If you are not drinking enough water, your body will store it making you "gain" weight. This is why you need to drink a minimum of 64 oz a day. If you drink sodas or coffee, you need to drink even more water as the caffiene in drinks will dehydrate you.

No one gains 5 pounds of fat over night. In order to gain a pound of fat you must consume 3200 calories more than you burn up. So that means you had to consume 16,000 calories the day before.

If you ate something saltly or with nitrates in it, you are probably retaining water.