R.H.
totally normal.
Explain to her that the area covered by her panties is private and she shouldn't touch it in public and noone except mommy (and daddy) or the doctor should touch her there and if someone does she should tell you.
When you catch her learning about herself (which is all she's doing) just distract her with something else. Reminding her that that's private.
It's a shame that it seems to be such a social no-no for girls to touch themselves but seems to go virtually unnoticed when boys do (0-99 years old! lol) We are all sexual beings and when something feels good we want more, weather we know the social standards or not.
hope that helps,
R.