I think the thing that was missed here is that the weight went down while the clothing stayed tight. While muscle does not weigh more than fat it does take up less volume than fat does - replacing fat with muscle will leave you weighing the same but occupying less space, just as replacing muscle with fat can leave you the same (or lighter) but occupying more space.

Also, keep in mind that carbs attract water - to the tune of 3 to 4grams of water per gram of carbs, if you've had a few cheats that can translate into lbs on the scale. These water gains will make you feel bloated and make your clothing tight - and they can show immidiately afterward, or many hours later. They can also last quite a few days.

I'd bet my money on a combination of the cheating, the bloating, perhaps a little fat gained and a faulty scale. Keep the water intake up and stick to the plan and get a new scale

-Nat