There's not much to do, except make it clear to her that she can wear whatever she wants, if it's clean. Otherwise, I'd sort of ignore it, as hard as that can be. It could be a growth spurt (which was always accompanied by some behavior issues in my experience) or a development spurt, or almost anything. Let her know you sympathize with being unhappy, but otherwise make it clear it's up to her to handle her unhappiness, you know?

I know it's hard, though. Especially in the morning (ugh).