Hello and welcome to Just Answer !
May I ask a question ?
Have you ever suspected that someone might have touched her there before ? Has she shone any other signs of any sexual abuse ?
That being answered.then.
Most young children will touch theirselves, when they learn it feels good. Be careful to explain to her that there is nothing wrong with it, but it should always be a private thing, in her own room, and that no one should ever touch her there except herself.
Boys as well as girls will touch themselves for a time, its just a matter of curiousity about their own bodies, and then when they realize it feels good. For a time it may happen more often. Just let her know its not a nice thing to do in front of people, because you won't want her thinking its wrong, or dirty.
As she gets older it shouldn't happen as often at least not till she is a young woman or teenager.