I'm definitely going with hormones here. Bipolar Disorder isn't a rapid change of emotions. It's a mixture of periods of depression followed by manic episodes. (The episodes of depression usually last at least three times as long as the manic ones.) It's not very common, either. The lifetime risk of developing it is somewhere between 0.5 and 1.6%. It's almost totally biological in nature and must be medicated, usually by lithium.
According to the DSM-IV, you have to have had met the symptoms for both depression and mania, and they have to be unrelated to another psychological illness, such as schizophrenia.
To be diagnosed with a depressed episode, you must have at least 5 out of 9 of the following symptoms nearly all day, every day for at least two weeks straight:
1. Depressed or irritable mood
2. Decreased interest or pleasure in all or almost all activities.
3. Significant weight loss or gain (change of 5% when not dieting, or increase/decrease in eating habits)
4. Insomnia or hypersomnia (Insomnia: inability to fall asleep, stay asleep, or waking up too early)
5. Psychomotor agitation or retardation (slower movements / very lethargic, or fidgety, uncomfortable, agitated)
6. Fatigue or loss of energy
7. Feelings of worthlessness or excessive / inappropriate guilt
8. Decreased thinking, concentration, or indecisiveness
9. Recurrent thoughts of death (in any way)
To identify a manic episode, you must have 3 out of the 7 symptoms for at least seven days and nearly all of the time.
1. Enflated self-esteem / grandiosity (superhuman, no consequences)
2. Decreased need for sleep - hyperactivity
3. More talkative than usual (pressurized speech - stumble over words)
4. Flight of ideas or racing thoughts
5. Distractability (attend to every tiny thing)
6. Increase in goal directed activity (work/social/academically driven)
7. Excessive involvement in pleasurable activities that have a high potential for painful consequences (no long term thinking: spending recklessly, gambling, promiscuity, substance abuse)