Is it possible to gain weight while on the atkins when your body is in the state of ketosis?

Well, I'm on the atkins and it's the end of my third day so I should either be converting into ketosis right now or I should already be in it. I only eat things with 0 grams of carbs in them so I know I need some extra fat intake for energy. I can't gain weight from eating too much food with 0 carbs in them right while my body is in ketosis? Including fats like mayo and er? I am so confused.