You seem like you're really looking for confirmation that you're some sort of badass for making out with her friend in front of her. I don't think you're over the girl who rejected you and were hoping comments here would see you as some sort of cool dude for making out with her friend right in front of her.

Feelings are complicated. Internet posts aren't strong on details. Idk, maybe you hurt her in the past. Maybe she is being selfish and just liked the attention and now is jealous it's leaving her. Maybe she had latent feelings that unlocked when you kissed her friend. Who knows. Can't tell from your post.

You said a month passed when she rejected you. That's really not a lot of time at all and clearly you guys had enough of a friendship and history to merit a post here about it. I mean your post isn't even about the girl you kissed, as cool as you try to act about it. It's actually about the girl who rejected you, so I think it still bothers you more than you admit.