I hate to beat a dead horse, as what I am going to say has kind of already been said. I just want to add my thoughts and perspective. I also find it very odd that she claims that she did not say anything to her ex about you. Why do I find that odd? Well, when you are in a true love relationship (and sometimes even a "like" relationship) with someone, you tell EVERYONE you know about that person. In this case, it almost sounds like she does not want to mention to her ex that she has met ANYONE, much less someone that she has been with for even 4 months. If I would be wondering about anything, I would be wondering WHY didn't she say anything to him about the fact that she even met you as a friend? She didn't even say that she met you PERIOD, right?
I don't think that you are a bad person. EVERYONE has (or will) go through some kind of heart ache in their lives. It means that we are human and have feelings. We are just here to offer you advice from our own life experiences. Take what you can from all of it, but don't get mad at anyone who may say something that you may not want to hear. If you don't like it, ignore it. You are going to do what you are going to do, regardless of what anyone says about it. Believe me, we care about your situation, otherwise we would not be commenting on it. We just don't want to see another human being hurt in the same way that maybe we have been hurt in the past by someone we THOUGHT cared for us as well. That is human nature, too.
I also wanted to comment on the fact that you say that you really know what this girl is thinking. Believe me, no you don't. No one can be 100% sure of what another person is thinking. Being with someone for just 4 months does not mean that you really know a person. Shoot, sometimes when you are with someone for over a year, you may not REALLY know someone. You only know someone as well as they WANT you to know them. I don't think that she has been totally truthful about who she is or her intentions with you or with herself. I just say be careful. If you want to go after her, then do it. However, know that past behaviors are VERY GOOD indicators of future behaviors. If you can live with ALL of her past behaviors and want to continue questioning why she does everything she does, why she says the things that she says to you and what she means by the things that she says, why she does or does not say certain things to her ex about you or anyone else, then go for it. No one is stopping you.