Six years old girl pushing her private parts against a pillow

My six years old does something before bed, and when she wakes up in the morning, that looks sexual. She turns on her tummy and pushes her private parts against a pillow.
She has done that ever since she was about one year old.
What is that?
She says that she feels good when doing it and does it when she is bored.
How to help her stop it?
She feels ashamed when I catch her doing it. But it looks like she cannot help it.
I would like to know as many details as possible about why and how to help.
Thank you,
Lucia