I think in order to answer that question, one might have to have a better understanding of how sexuality works! What causes attraction to men? What causes attraction to women? Is it a mental change? A hormonal change? A chemical change? Something in your circumstances at the time that altered your ability to perceive one or the other? I don't know. I do wonder though that if your sexuality changed because of you being bipolar, it would fluctuate much more frequently than it has.

It could just be that that is how your sexuality is. Not all bi/pan people are attracted to men and women equally throughout their lives- from what I hear, most people do have a bias towards one or the other and can change like you have.