I agree--it's pretty much impossible for you to have gained that much fat in a week, unless it was a REALLY bad weekend...
More likely, you're retaining water. It could be from eating foods with a lot of salt, or because of TOM, or because your metabolism shifted from the foods on the weekend. As others have said, get back on track, drink plenty of water. Drinking water helps to flush out the salt and the excess water goes with it.
It's not unusual for water weight to make that big a difference.
Good Luck!
Jay
P.S. 8 to 10 8-oz. glasses a day is plenty, by most sources.