You are trying to make a decision based on a look she gave you? And why keep saying that look was not only for you? She looked at you that way, who else was in the room? It was for you. You do not how how she looks at the other guy, nor all the details of her relationship with him, so why try to compare what you have with her to that or anything else? It seems that you enjoy her company and feel good being with her -- until your mind starts fucking with you. The issue is not her practice of polyamory, it is dealing with your thought processes and responses to your conditioning about monogamy and all that you expect from a partner. To me, the question you should ask yourself is, "Is how I feel when I am with her worth challenging all the conditioning and ideas I have about what a relationship should be?" Maybe if you test the limits of your comfort zones, you will find that you are okay with much more flexibility in loving someone than you previously thought. It is possible that a relationship which is radically different from what you think it should be can hold gifts and teachings and value for you far beyond any expectations you might have, especially if you have a kind and loving connection to that person. You might learn something about yourself and about other people in the process, and learning is always good for us.
The world opens up... when you do.
Oh, oh, can't you see? Love is the drug for me. ~Bryan Ferry
"Love is that condition in which another person's happiness is essential to your own." ~Robert Heinlein