I am 25 years old, and what you're describing sounds like a common thing for me these days
. I tend to burn calories easily, so I have been known to lose weight very easily if I don't eat enough...even during a week's time
. My family and friends are pretty good about letting me know that I have lost weight, so I rely on them to keep me in the know about whether my appearance has changed. My mom and gram are especially aware of this and will say, "Eat! Eat!"
From my own experiences, I think that your body may need the extra food in order to regain any weight that you may have lost in the process...even if you were eating things that were nutritionally dense. Do you have a high metabolism?