Most of the times, it is hard to be certain that the disease is behind the behavior, but you know she suffers from it, and if you educate yourself, you should know what sorts of behaviors it should cause. Especially if she is showing symptoms of an episode, and the behavior is consistent with common behavioral patterns during episodes, it is best to assume that it is the disease. Give her the benefit of the doubt that it is not normal behavior. If she is still telling you those things in six months, well, it probably is how she feels, not the disease.
It took me almost a month to come to terms with the fact that the disease could cause this sort of behavior. In my case, it was more obvious, because I knew it was simply not normal behavior for someone to start telling me that she was not sure she loved me after telling me how much she loved me for the past two years (without providing a reason). In your case, well, it is unusual but not necessarily abnormal behavior. Sometimes women need their space. What is telling here though is that she does not seem to be able to provide a convincing explanation as to why (which is consistent with mood swings caused by chemical imbalances) and her behavior is consistent with a mood episode.
It is possible she is manic, and difficult to guess, because you left a lot out, but social withdrawal (including from close friendships and relationships), feeling overwhelmed by social obligations, crying, et cetera, are all behaviors associated with depression. You might want to consider whether she is showing the common symptoms of either mania or depression.
Finally, something I wish I had known a few months ago, you need to watch out for times when you find yourself arguing with her bipolar disorder instead of with her. If you are, just stop. Providing logical deconstruction of something said in the midst of a chemical imbalance might be effective, but arguing with the disorder just makes it worse for both of you.