G.♣.
My first thought was, 12, hmmm, that's when I first got my period.
But I always hated it when people would assume if a girl is having a bad day, she must be on her period.
Really, she's 12 years old, and personally, I hated that age. There was so much drama at school. Girls were really mean, and it was hard to fit in.
Talk to her. Take her out for a coke or milkshake or something, just the two of you. I wouldn't ask her directly why she's been so emotional lately, but I would ask her how things are going - school, friends, activities, etc. Just give her a chance to share with you what's going on. Listen. Really, really listen. If there's something going on, you want her to feel comfortable talking to you about it. It might take a few mother/daughter outings to really build this relationship, but it will be great for both of you.