Yup, I'm with Cassie and have tripped myself up sooooo many times. Don't manage the food mid-day and pay for it at the end of the day.
So many times I have tried to convince myself that I am just not that hungey at lunch and therefore I shouldn't bother to eat (or eat much). But pretty much every single time I pay later in the day.
It is like learning to eat breakfast when you aren't really a breakfast eater (that would be me). You just gotta do it most of the time otherwise the hunger monster will catch up with you sooner or later.
BUT as CJ says, Zoe, that was a pretty minor slip up. Most of us have guzzle whole pizza's in that situation. So good on you girl. Shake it off, learn the lesson and move on!