What's the difference between bipolar II disorder and teen moodiness?

See your doctor. Mood changes in bipolar disorder tend to occur without cause or are a exaggerated response to an event. The individual tends to expresses frustration with them selves, often saying "i don't know why I react this way. Moodiness in teens tends to be persistent and is related to events. Moodiness can also be depression. The best thing to do is to consult with your physician.
It's episodic. Bipolar disorder is an episodic illness. It comes and goes often without cause. If your teen is always moody when told no, or is moody every day, it is less likely to be bipolar disorder. If, however, they go months without issue but then are agitated, moody, and a different person for a period of time, that is of more concern for bipolar disorder.

Related Questions

Can you explain to me the major differences between bipolar and bipolar II disorder?

Bipolar. Bipolar 1 is manic depression with phases of depression alternating with manic episodes. Type 2 is less severe, with hypomanic phases.
Mania vs hypo mania. Both usually have depressive episodes. Bipolar 1 includes manic episodes and bipolar 2 hypo mania. Hypo mania may include decreased need for sleep, increased self esteem and increased productivity and sometimes irritability. Mania often leads to psychosis, severe relationship, financial, or even legal problems and often requires hospitalization.

Bipolar II disorder or BPD...how can I tell the difference?

Bipolar. Bipolar II disorder can be thought of as mood swings from deep depression to a hypomanic (which means not fully manic) state. BPD or borderline personality disorder is characterized by a disturbed sense of self, impulsivity, manipulativeness, thinking of people/things in all good or all bad terms, intense fears of abandonment, sexual/substance use, risky behaviors, and self mutilation.
Either-possibly both. You're raising a difficult question, in part, because both can be present and you may need the help of a professional to sort it out. But, in very general terms, BPD mood swings tend to come on suddenly, last for a period of hours (rather than days/weeks), and often clearly in response to a perceived abandonment. Bipolar patients often say their moods make no sense to them, but can't control them.