Why does my 7 year old girl ?

My daughter started a pillow when she was just a baby. I didnt really think anything of it so we just left it alone. Then here recently she has been I guess . She puts her hands down there and kinda humps them. This happens at least once a night or every other night. I always have told her that it wasnt nice to do that bc she would do it when people are in the room, like me or her brother but wont do it in front of other people. So last night she asks me about it and says mommy you know how you told me not to do that thig with my hands well I need to because my private makes this feeling and I dont like it and I just have to do it to make it stop??? I asked her if it hurt or anythign and she said no. But its kinda concerning me. I dotn know if this is normal or if maybe she is maturing too fast???? I know she hasnt seen anyone do this and doesnt know what is or for that matter.