It sounds like your gf is a rapid cycler, meaning that she can shift from manic to depressed several times a day all throughout the day. There's quite a difference from mania and a girl just acting like "she's in a mood" you should be able to tell the difference. I would seriously hope she's not playing the "BP" card on you. Generally when one is manic they are happy, giddy, talking fast, thinking really fast, changing ideas constantly, jumping around perhaps, things of this nature, when one cycles into depression there can be quiet, sadness, sleepyness, insomnia, loss of appetite, things of this nature. Hope you can get it figured out. As one who has a seizure disorder and BiPolar myself, I know you don't need the stress of trying to figure this out.
Good luck and take care.