[Daniel-X]

You have every right to be annoyed don't feel like you're being childish. I would be super pissed also if someone did that to me. She could have easily told her mum she wasn't going to this party or invited you if she felt this party needed to be attended (Was it a relative or something I'm guessing?) Don't dwell on it too much and just focus on other things right now like your other friends if she apologies or try to speak to you fair enough I think you should give her the benefit of the doubt (but that's just me it's up to you whatever you do)

A similar thing happened to me when I was sixteen and it was prom one of my best friends told me she couldn't get in a limo with us or get dressed with us before hand etc because her mum couldn't drive her round to mine yet her mum could drive her round to someone's which actually passes my house. We made up but I was really pissed like you at the time.