My daughter does that because she's sweating under her nappy. The part of the nappy that covers her lower back feels like plastic and she's frequently sweaty when I help her scratch. I noticed that wiping the area with wet tissues, putting on some cream and letting her without a nappy for about 10 minutes once in a while helps.
My little girl has started scratching the side of her thighs near her bumcheek to the point it's all little tiny scratches, again I don't know why she is doing it although I have noticed she does it when she's tired aswell but her skin looks fine so I'm not sure
My daughter has what looks like a heat rash at the very bottom of her back and on her bum and like Selda said I'm pretty sure it's because it was really hot and she was sweating and the nappy had irritated her skin. I plaster her bum in sudocrem and that helped!