Ask for pads or tampons; it will let her know you had your period without having to tell her "hey, mom, I had my period" which sounds really embarrassing. And, you kill two birds with one stone, since you also get the stuff you need.
Well, my parents were divorced and I wasn't close with either of them at the time. So, I was staying at my dad's house for the weekend. I got it on that Friday. I didn't know what to do. I was mortified. So I waited until Sunday, when I was going home to my mom's house. I ran in the house crying! My mom asked me what was wrong. I just kept saying that I didn't feel good. She guessed what had happened after a minute. It was so terrible. Oh, did I mention I was only 10 years old? Yeah. That was the worst part. I had no idea what was going on. I will never forget that... Scarred. For. Life.