How should I tell my friend this?

well one of my bestfriends hasn't been actin like a best friend lately she started hanging with some new friends and she act like I'm not even here I always have to be the one to ask do she want to hang out and stuff and one day I was on facebook and her and her friend was commented on each other walls so I said you can't hang with anyone no more and she said well you don't be asking and I said I shouldn't be the one that has to ask all the time and she said wel and I didn't reply so I been making new friends getting closer with them and stuff so me my sister and my friends amber and chelcia we made a plan to all go to the beach today and I invited my bestfriend and I told my sister and se got kinda made about it cause the way she be treating me so now I got tell my friend she can't go but idk how to tell her I need help please