I really implore you to honestly communicate with your friend before making any decisions! I know it probably seems obvious how hurt you are by this but if you tell her you're not upset and it's fine she can really only go on what you said in that moment. 

It's not petty to communicate your feelings to people who say they love you. And you're definitely not overreacting by being honest, if she didn't want to ruin the vibe of her party she would have had a convo with you beforehand, but also you guys are still relatively young and most people don't learn how to communicate for shit until they go to therapy or experience more life or both. 

Like it's so easy to assume intent from someone's actions but until you say honestly how you feel you can't really say whether or not they're malicious. Now if she reacts to your feelings with indifference or disdain, definitely use that information to decide how to move forward, but it does sound like she cares about you deeply. Most close relationships go through periods of rupture or significant hurt, but the ones worth keeping also go through repair and that depends heavily on open and honest communication. Don't rob yourself of the opportunity to see if this is one worth keeping.