I was super nervous too and it took a while for me to get enough courage to ask I told her I thought I started and she was super casual but excited. It was late at night and she said ok lets go buy you some stuff and we got a lot of stuff. Anyway my point is there is nothing to worry about she will be happy for you even though your period kinda sucks.

You should just act casual and open with it. Just say "Heyy Mum i just wanted to ask you a question about periods cos i got mine today..." well something along the line of that. The more open you are the better you will both feel :) xxx