Loquacious72 Yeah, you will obsess for a while; it is just natural. You are trying to see what she has that you don't; and why he would try to win her rather than behave with you. There are some very deep answers to these questions, it has to do with his self esteem and his belief that he can't keep you so he has to make do with whatever.
As for her giving you a dirty look, my ex's new girlfriend did this too, she tried to get a harassment order against me because I told her what he was really like deep down inside, a cheater and a liar, but she chose not to listen. The judge denied the order and told her she probably should listen to what I was saying, and I was saying it to help her. So she actively hated me for a little while. Then, about three months later, she was calling me from a woman's shelter, crying and telling me she wished she had listened to me. And now we are, well, not friends, but I don't hate her by any means.
And that is probably how it will go for you too.
Keep cool; it will all unfold in a natural way.