I got my period when I was 10, and cried hysterically because I didn't want to grow up. I felt like I was the onlyyyy one who had it. Turns out a lot of my friends got it around that time too, and I didn't even know!
As the person above me said, tell your daughter that it is normal and just a part of growing up. I wouldn't say you need to give her "the sex talk" if you don't think she is mature enough, but atleast give her a "puberty talk". Because changes like getting your period, growing armpit hair, etc are a lot less scary when you know they are coming.
