The NOS categories are used when you do not fit the description of one mood disorder. For Mood Disorder NOS, you may have symptoms of anxiety and bipolar, for example, but not enough of either to make a diagnosis.

Mood Disorder isn't a diagnosis because there is more than one (Bipolar, Major Depressive Disorder, Major Depressive Episode, Cyclothymia, Dysthymia, etc) and Mood Disorder is a category of disorders. If you have a diagnosis of NOS, the practitioner should really specify why they gave you that diagnosis by noting your symptoms. For example, if someone has Personality Disorder NOS, the practitioner should put Personality Disorder NOS with borderline and histrionic traits.

OP, you may have been diagnosed as Bipolar and they put Mood Disorder because that's the category it falls under in the DSM-IV-TR or you were diagnosed with Mood Disorder NOS with Bipolar traits. Either way, they should treat you and medicate you as Bipolar.