I don't think I'm my best friend's best friend

I really wanted to vent... Yesterday there was a birthday party for one of our friends, and he only invited about five close people. Anyway, when I got there, my friend was kind of distant and barely even talked to me. She spent the whole time with another girl , they kept going to the bathroom to take pictures and just left me alone with the guys. It felt like she didn’t even know me.

At one point, some of them started exchanging looks and laughing... I thought it was weird. Later, I saw on a friend’s phone that they had made a group chat. I don’t know what it was about, but they were talking a lot in there. I just thought it was strange that my best friend didn’t even tell me about it.