Is there a reason why she change her look?

Alega said:

Is there a reason why she change her look?

Her "high school debut".

Her reasoning for changing back will be revealed in one of the upcoming Lite episodes.

A shame, while I love the original Nibutani the new look gave a really refreshing look to her. I really wouldn't mind her keeping the black hair, but only as long as she wears the regular skirt instead of the long one.