Most Helpful Girl
I think it could lead to a fear of commitment. But, honestly, if she really loved you, she wouldn't have put you through this and won't fall out of love with you that quickly. It seems that you are willing to invest more into the relationship, but she isn't. I think its best to just move on and find someone that will treat you better and won't change their feelings towards you week after week.
Most Helpful Guy
It's been said that woman who hold an extensive list of sexual partners tend to have trouble committing to one person, one relationship, for an extended period of time and therefore have a greater chance of cheating or moving on.
Being that you are aware of past, why, for crying out loud, would you be surprised of her decision to move on? Why did you ignore her behavior patterns?
In any event, she could possibly have a fear of committing. Or, it's possible she simply does not want to settle down, which may have nothing to do with fear. And more to do with her personal preference.