Is it possible some scary things may have been said to her or she has seen on TV or Videos?How does she get on with her older siblings are they kind to her,Does she sleep in her own Room.Ask her why she is crying and find out if anyone or anything has happened recently to upset her. As she is a good student she likes school and nothing happens there, something is disturbing her ,observe and you may find out.
She gets along great with her siblings and doesn't watch a lot of TV. She spends a great deal of time outdoors playing. She won't talk when we ask why she's crying. She calls herself stupid. During the daytime she's fine. It's when bedtime hits that the problems start.
Is there a history of anxiety in the family? The behaviours you have described are common to anxiety issues. I'm assuming school has started (or about to start). Could this be the problem? If anxiety is the issue, she will not know why she is crying or why she feels so blue - but depression is often co-morbid with anxiety issues (as is low self-esteem and perfectionism). Another possibility - is your daughter being bullied? This, too, can cause anxiety problems. Perhaps her teacher might be able to shed more light on the issue. Just a couple of possibilities ....
It sounds to me like she is experiencing verbal abuse somewhere. How does she get along with the other kids at her school. I ask this because my daughter used to act the same way and when she was 12 I found a suicide note in her room and had to take her to the hospital for observation. (the hardest, yet best thing I have ever done for her) She was picked on at school so bad and the other kids would tell her she was stupid and fat. When you spend 8 hours 5 days a week w/ people telling you this you begin to believe it. The good news is my daughter is now 14 has lots of friends and just started high school. She is an A student and does not let what other people think bother her anymore. After being released from the hospital (after 2 weeks) she was put on antidepressants for 1 year and continued counseling for 2 years. I wish I had not let it get as far as it got. Talk to your daughter when she is not upset and ask her what is going on and who is calling her stupid. Deal with it now, so you don't have the scare I did.