I think the 3rd door meant, that there could be someone else, not that there is...because you never actually saw her go through it did u? I also think this is your deep rooted insecurity from your ex. This really should be dealt with though, because if she were to ever cheat( im guessing you both are pretty young) that it could cause more trust issues in future relationships, which can be destructive and become a cycle. One thing I can tell you is this, I have been cheated on, I dont cheat, but it is not your fault or your shortcoming, there's nothing wrong with you that made her cheat. There is something wrong with her. She was the one who was being sneaky, lying and has questionable morals. Dont carry the monkey on your back, put it back where it goes. good luck