The Bipolar FAQ sez:

-- Bipolar I is the "classic" form of Bipolar Disorder. It most often involves widely spaced, long-lasting bouts of mania followed by long-lasting bouts of depression and vice-versa. However, the essential definition is depression plus mania, or "mixed states."

-- Bipolar II involves at least one Hypomanic Episode and one Major Depressive Episode, but never either a full-blown Manic Episode or Cyclothymia. The essential definition is depression plus hypomania.

With bipolar II, I never really get manic...you won't find me going on shopping sprees, binge drinking, or racing around town at 130 mph! But, without meds, I do have pronounced, sudden swings between debilitating depression and feeling just fine.

darkeyes has good advice...a good psychiatrist can get to the root of the problem, which could be any number of things. Good luck!