I read a post from a respected user on another forum website(bodybuilding), that she believes that a woman's metabolism actually increases in the third week, which is what ramps up our appetite.

My unscientific, unthought-out or researched (as of yet) idea is that our bodies are preparing for the loss of nutrients ahead of us in menstruation, so our hormones and chemicals are somehow triggering a desire to have a greater nutrient intake. Like I said, very unscientific!! but a maybe?

The reason I am not sure about the metabolism thing is because I've read from a few different sources that post menstrual and ovulation was when we have the greatest fat burning capacity, the greatest physical strength and best performances in terms of physical and mental ability.
There is even some reference to one the articles in the exercise forums somewhere...

Meg

...munching on homemade carb free gelatine gummies as she does her homework..... and gets distracted by her email program