What Could Cause Sudden, Unexplained Sadness With Loss Of Appetite, Sleep Problems, And Headaches In A Teenager?

My friend has a 16 year old daughter who cannot stop crying. This sadness she describes is not founded in an event or anything she can link to and she can t say why she is sad. This sadness causes her to feel as though she can t eat; and she is not sleeping. Blood work has all come back normal. She is experiencing some headaches, but it seems the sadness is the precipitating factor inducing everything else . She will say she is not depressed about anything, does not feel overwhelmed, or anxious. This started about 30 days ago, without any contributing factor that can be isolated. Endocrine system maybe? Not sure where to start.