I have not had the experience of this.
But she doesn't know what to do with her, she does it constantly wherever they go. Anywhere they go, her hands are....um...."down there".
Has she done this since she was little? If so it is probably still it just feels good. She is 5 and should be able to be told about the fact that this is only done in private. If she has just begun I would defiantly check it out with her pediatrician. It could be as simple as a yeast infection or even strep. Both of these infections itch and can seem like it is her doing some thing that she is not doing.