you  need to sit down and  talk to her.....tell her to come clean if she is hiding anything, tell her whats bothering you , tell her everything you wrote here, Im sure there is an explanation, sometimes some relations are too bitter to admit or even remember. Sometimes people erase their past for a new beginning  and Sometimes imagination takes the best of people to become irrational. the more you keep this a secret it will ruin both your lives. May be its not what you think.....but instead of asking for advice here, you need to sort out your own demons have a clear mind and then talk with her. You might hate the outcome or even love it, cant live life like this bro.......clear open communication can help you and her....GOODLUCK