DS went through a similar thing. I think its about the time when they start to realise what the emotions are they are feeling. I think the best thing you do is tell her it's ok to be sad sometimes. And show her ways of cheering herself up. I do this with DS who is still going through a very emo phase. He is extremely emotionally sensitive, and I do not want him to think he is not entitled to his emotions. But often the reason why the sadness perpetuates is because they think it's wrong to be sad, or that there is something wrong with them to be sad so it goes in a cycle. Teaching coping mechanisms and giving their sadness a voice is the best you can do xx