curly to straight?

My hair is all one length and it's almost BSL. Lately i've noticed that it's been growing in straight up top (about down to my ears), and then it gets curly. At first I thought it was just the weight holding it down but since naturally curly hair is textured I would have thought the strand of hair would still be thick in some places and thinner in others, and it would just be straight. The thing is that it's all one thickness. Could my hair be growing in straight?