There is something called carbohydrate intake. It helps to make your body stronger and produce energy. While you are doing those sit ups, your body isn't used to the sudden movement lately and it starts to use more of the carbohydrates to replenish it. Your body isn't trying to make your stomach chunkier, its trying to help it before it becomes malnourished.
Not only sit-ups will work. You need other workouts as well, for example, push-ups, lift weights, running a mile ( it's not as hard as it sounds, I run a mile and a half every Friday non-stop in 6 minutes).