ricks,
Your previous question was "How to make my Girlfriend cry"? You had said there, that both you and your girlfriend are in a serious relationship. So what has made you change your mind? By doubting your girlfriend, you will surely make her cry and fulfill her wish and the oath you had taken in your earlier Question.
I think your girlfriend has been very honest with you and given you all the information you need. Believe me, once you start a relationship on a doubting note, it will continue all your life and and even a stray incident will put doubts in your mind.
You do not own your girlfriend, and you have conveyed your unhappiness with the password, which I presume you have changed to your name. It would be strange on her part to stop all contact, with the man she was going to marry. If she had anything to hide she would not have shared her pesonal Social ID with you. The loving terms that you discovered was during the time when you were not in the frame, so you should not object or have any fears about that.
Let the past, be the past. Look to the future. Give her mental support when she needs it. All the best.