It's not uncommon for teenagers to have mood swings, especially
under alot of stress. In some cases, some people might develop
bipolar problems, and their mood swings are extremely
unpredictable, and they can get very intense without any apparent
reason.
http://en.wikipedia.org/wiki/Bipolar_disorder
But most of the time, even if you don't understand what the reasons
are, it doesn't mean that she doesn't have one.
