You can sure her some fun educative video on it.. tell her not to get scared that time.. and that its pretty normal. . She should not feel ashamed about it.. tell her that all girls get it.

You have talk to her and also show small videos ...talk to her on this and that it's a natural phenomenon of the body...

Hey dear, aap pehele se hi usko educate karo, ki kya hota, kyon hota hai and jub bhi peroids aye toh kya karna chahiye