Girlfriend with friends on new years eve?

Me and my girlfriend have been together 9 months, I am 18 and she is 17. She just said she is going to a house party on new years eve with her mates. I don't really know these mates so it would be wrong of me to just go with her. Is it right of me to feel a bit upset and hurt that she is choosing her mates over me for the new years eve countdown?