I wonder why she kept it? Then ...
I wonder why she lied about it?
I also wonder why she bothered to tell you about it if she was going to lie about keeping it?
You know her a month so I doubt she's going to shut down all of her options at this point. Have you two had any kind of exclusivity talk yet? If you haven't because you think it's too soon then you should keep your own options open as well until you can see in her actions that she's ready to be exclusive.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion