Why is it bad to lose more than 2 pounds in a week if all you're doing is eating healthy?

I've heard from different sources that you shouldn't lose more than 2 pounds per week. I started eating a lot healthier than usual...eggs, oatmeal, veggies, fruit, lean meat, whole grain cereal, only water to drink. I've lost 4.5 pounds in a week. Why is this bad?