I've had the same thing happen to me, except I'm going from wavy to straight. I think growing it longer has affected it some, and also, my age. I had straight hair until about age 12 when it got very wavy with some spirals. As I'm getting older, it's reverting back to straight, I think. It is still wavy when it's very humid, but otherwise, is only slightly wavy in back with straight hair on the sides.