Welcome to healwell's bipolar forum. I would think that since you are both bipolar you would be able to understand each other's moods a little more.
Firstly, are you both seeing a psychiatrist regurlarly and taking your medications?
Things must not be right for her since she is self medicating. Either the meds are doing it's job or she is just trying to push her saddness further down and out of her mind.
If so, maybe you both need to get involved with a therapist. Maybe you girlfriends trusts issues are more deeper that labeling it as bipolar.
I don't like how a lot of people try to rationalize all of our behavior traits to having bipolar. Lots of times our feelings and actions are justified to what we are dealing with in our lives.
There must be a reason why she doesn't trust you. Did you do something to her that would cause her to feel this way? How would you feel if the shoe were on the other foot. Would you have questions and feelings that would bother you about a situation like that coming from her?
Just some things to think about.
Hope things work out well for both of you.