oh sweetheart あなた gonna have to tell her! And she will help あなた its not like she is going to get mad.. but あなた should say "mom i started my period what do i do?" または something like that lol. when i started i freaked out and ran and told her so yeah, i hope that would help
Just straight out tell her. "mom i got my period". She wont get mad または anything, its embarrising i know, but shes been through it and knows exactly how あなた feel. Plus its a good thing! Your finally a woman(: