Thank you so much for your question. It is my privilege to be able to help you.
The first thing you need to determine is if there is any possibility that someone is abusing her sexually. If this is at all possible, it needs to be in vestigated. If you determine that it is not possible, you should consider several things. One is that she may have some irritation there, this could be caused by frequent masturbation, or it could be caused by irritation from soaps, shampoos etc.
She also may have a foreign body placed in her vagina, which is very common at that age. It is important to have her seen by her pediatrician for an exam.. Usually it is obvious on an exam what the problem is.
I hope this was helpful to you. If it was, please remember to ACCEPT the answer so i can get compensated for my work. if you have any more questions, please feel free to ask. i am truly privileged to help you, and i want to make sure that you are satisfied with your answer. Good luck! Dr. SaulJE