As someone who has been incorrectly diagnosed with BP, and seen others be incorrectly diagnosed, I would say to get a second opinion to ensure that it is in fact Bipolar Disorder, and not something else (I actually just have depression and anxiety).

It is very much an overdiagnosed illness in my opinion, which is sad because it can negatively affect treatment by giving the individual the -wrong- type of treatment.

Aside from that, I would echo what an above poster said: A bad day does not mean a depressive episode, nor does a good day mean a manic/hypomanic episode.