THE PEOPLE I KNOW- The Film Noir Princess

So she is quiet, oh so quiet. But she does speak, telling me this and that about interior design and the parts of her thoughts that she deems safe to be known. The rest is kept in her mind.
We go to lunch together nearly every day and she knows I adore her. We talk and I tell her truthful honest things about my life. She was cagey and timid to begin with but I enjoyed it and played along with my little film noir princess. But then I felt as though we were making progress, felt as though she was opening up to me and that she trusted me as a friend. All was well for a while.
Then 'he said' 'she said' shit happened and I find out she slept with someone we both know on more than one occasion. To analyze my feelings on this I cant be succinct, I must explain thoroughly for my own peace of mind. I wasn't angry because there is really nothing to be angry about, she is entitled (as we all are to keep things private). If anger wasn't the thing bothering me then what the hell was it? Pinpoint fingers on lips and voila! I come up with it. It's because she DIDNT tell me that it bothered me, its purely because there is something pretty big that was going on with her and she decided that I was the person who should know about it. I felt as though this put some kind of confinement (limit if you will) on the level of our friendship 'theres only so far you can go with her...' and that made me feel pushed away. Umming and arhhing 'should I say something?' 'No what would be the point, it would be petty, she has rights to keep things private and how would I look if I were to confront her about this minor detail?' I decided against it because it almost felt intrusive to mention it, almost felt as though I have read her diary, finding out something that someone doesn't want you to know about them is tresspass of a sort isn't it? Even if you purchanced upon it quite innocently, someone told you and you couldn't help it?
She is a strange little cagey creature and although I haven't mentioned it and I am no longer too concerned about it, she perplexes me and maybe thats why I like her.