I would take her to the GP. She might have a vaginal infection like thrush or something. Maybe she is itchy. My cousin had heaps of issues with that sort of thing from a very young age (well before school age).

Take her to the GP. Find someone with a Lady Dr & ask when you make the appointment if they are good with kids.
The medical center I go to, each Dr has a little write up about what sort of things they are interested in. Find your self a good GP & stick with them But yeah I would be wondering if there is more too it then just she enjoys it.
My Daughter has never done it & to be honest, at 2 / 3 yrs of age its not something I would have considered normal. Not this much anyway.