Why is my skin more dry since giving birth?

Dehydration. If you are nursing (breast feeding) then you'll have increased water intake needs. During my wife's pregnancy we used to use a 5-gallon bottle of delivered water (you don't necessarily need special water, but it helps for this analogy) every 2 weeks. After she delivered and was nursing, we used 3 of those bottles every 2 weeks! Dehydration can lead to dry skin. There are also hormone shifts!