My guess would be that you haven't been having enough to eat for your balanced meal..... the guide says your plate should be 1/4 meat and 3/4 veggies... would that be about right for what you had tonight?
Also I dont think one extra snack in one day is going to make a huge bit of difference to your overall weight loss in the week, I think the allowable snacks work out at 150 - 200 cals max which you more than worked off on your walk today.
As you said, it could also be habit, so stick it out and see how you go. x