When you change your diet and activity level, your body is going to be going through chemical changes.
If you aren't eating enough, or you go through withdrawal from all the extra sugar, carbs and fat you're used to eating, you will be moodier.
Stopping caffeine in things like pop can cause irritability.
Changes in your blood-glucose alone will cause you to become moody and irritable, as will lack of good hydration, so make sure you're eating evenly-spaced meals/snacks and drinking enough fluids.
It's not a matter of skinny = mean, rather
getting skinny = irritability sometimes.