Unless you ate 7000 extra calories, this is not "fat" coming back. It is a rule of dieting that scales fluctuate. They fluctuate for all sorts of reasons, but water retention is very common. Maybe you were retaining sodium or it was TOM? This is one of the reasons why weight loss is not linear.
Also, did you weigh at the same time of day with the same clothes, etc? That can make a big difference too. Most of us weigh less in the morning. My official weight each week is in the morning, after bathroom, undies only. I weigh at other times, but never count it. I observe the fluctuations but don't let them bother me.
I am also of the opinion that scales are

They hope their fluctuations cause you to go off plan -- and you did!
You need to worry less about what the scale says on a day to day basis. This is why some people don't weigh everyday, but you can see these fluctuations even when you weigh weekly. Just don't worry about it. Focus on your behaviors -- and the weight will come off!