I think just go up to her and ask what a period is and talk to her about it even though you might be nervous just go for it and your mom will help.

I will tell you 2 options

1. You could send her a text message as it is easier when you are not face to face with her so start by texting and you can complete by texting or tell her in the middle that you are bored of texting so you will ce to her and complete talking (this is the easier choice)

2. You could do luke this

You:hey mom, do you know Sophie my friend

Mom: yes i know her

You: she just got her period

Mom: oh, really

You: (and start talking or asking your questions)

Good luck 😃❤️

When I first began to talk to my mom, I would be quite nervous. But my recommendation would be to sit down with your mother and just talk to her. It will get easier as you keep talking. Don't worry. Your mother went through this stage before you did, so she will be glad to help!