That's the carbs finally leaving your body. It takes a few days. A lot of the weight you lose right off the bat is the extra water your gut needs to process carbs. This is why fiber (what retains the water) is so important in carby diets, but not at all in no-carb diets. You'll also notice needing to use the restroom less frequently. This is just your gut needing lower mass flow for the same nutritional value.