Although, mostly correct... The above is mistaken in some areas. Split personalities can and often do have awareness of "others". Needless to say that no one can from your description make an accurate diagnosis; the most likely candidate, though I hate to even breath it, is likely abuse in my opinion. Bipolar, schizophrenic, or split personalities are all so uncommon, you almost would have to seek them out to find them. It may be also any number of other things, please have her checked out if possible. I however am a scientifically minded individual, and believe the most likely is the first logical solution. It is a sad reality that sexual abuse is as or more common an affliction than most other mental illnesses likely much more. So I would look there before going to the extremes. If it is a chemical disorder, then there are drugs that work for a considerable number of those sufferers. A disorder such as this should never put you off from what you feel for them. Whatever the reason, it is about seeing her through this. Together for always, or just for now, how do you want to be remembered in her life?
Split personality disorder comes under schizophrenia. Bipolar is a type of depression.
Split personality I don't know about so well but if someone suffers from it they have 2 or more different personalities where they seem to be a different person acting in different ways, one personality may have no control at all over the other one and they may have no memories of what they did or where they were at the time when they were 'the other personality. (This has happened to some criminals who have commited serious crimes when they were one personality but have no memory of it or would ever repeat it when in the other personality.) I have heard that in treatments they try to bring the 2 personalities to 'meet' midway.
Bipolar/manic depression is a depressive disorder where there is only one personality (so differs from split personality) where the person suffers from extreme mood swings (mania and depression). When manic the person may have symptoms like high energy levels, increased sex drive, being unable to keep still or concentrate and feelings of extreme happiness. When depressed the symptoms can include extremely low mood and self esteem, lack of energy or motivation, lack of sex drive and suicidal thoughts/feelings.
A doctor/psychologist has to diagnose these disorders. Hopefully the answer to why your girlfriend was acting like that was just down to hormones, everyone gets mood swings sometimes!
'Bipolar' is a mood disorder, which may be caused by depression, repressed memories or a breakdown in the nervous system. Split Personality is having a different personality/ possessing multiple personalities inside one person, but the ACTUAL person has no memories of his/her alter-ego