Could be mostly water, especially if you've been low carbing for a while. Carbs attract water/are stored as mostly water in the body (hence their name, I never realised), so if you've been cutting them out, when you do have them, they rush to the liver and muscles and you retain water (it's actually more complicated, but that's how I understand it).
One will almost always be heavier when carbs are a part of the diet (in terms of numbers on the scale) but it's not necessarily fat. Of course, the type of carbs you take in matters too. The extra weight will be the first to drop should you decide to cut carbs again.