I agree with Donna, that it could certainly be hormonal. I started developing at age 8 - the doctor first thought I had breast cancer and was ready to do surgery on me! I think nowadays that girls are hitting puberty earlier and earlier. I would take her in for a physical exam and tell the doctor what's going on - they should be able to do some blood tests and figure out if it could be hormones or something else. I also was depressed before I was 12 years old, so that is a possibility.
Once you get connected with the right professionals, I hope you'll feel more at ease about all this. I'm sure you're very worried, but there is help for whatever it is. Both of my sons were depressed by age 12 and I know how much I still worry about them and they're in their 30's now.
Take care and let us know how this turns out.
Just a thought: I had my first period at age 9 so it is not out of the question for young women to go through puberty early. You don't say whether she is overweight, but overweight girls tend to hit puberty earlier than their slender peers. Something about estrogen being stored in fat cells, I think, although I could be wrong. I started having noticeable depression at about age 12, so that's not out of the question either. Don't be afraid to ask her if she is depressed or having problems...maybe she will open up to you. I often wished an adult would have asked me.
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My period started at 9 and THEN the obesity set in; not the other way around like I see suggested on this site.