It would be important to notice if these mood changes occur some of the time (she is too young to have her menstrual cycle) but if she has it, that may be something to mention to her OB doc. It may be hormonally triggered. If she does not have her cycle, notice if the behaviors occur as a result of some sort of conflict or when she does not get her way. Then it would be more of a behavioral issue versus emotional. If there is a history of depression in the family, then, an evaluation and therapy would be something to look into. Although, medication may be the last resort since she is so young.
Ask her to let you know what may be some of the things that make her sad, excited, what her short term goals/aspirations are, etc.