Am I overreacting about my friendship?

I (f17) have been best friends with another girl (f17) for maybe 7 or 8 years already and we have spent almost every Saturday together but recently she got a girlfriend and always seems to be with her no matter what. At first it wasn’t a problem but then it seemed that no matter what when I would ask her to hang out she would blow me off due to having to see her girlfriend. Recently I asked her to come to my younger sisters birthday party to hang out with me due to the only time me seeing her this month was for an hour and that was because she was drunk with her girlfriend and came to my house for her old vape though she said she wouldn’t be able to come cause her and her girlfriend had plans already which caused me to get a little upset with her. I just want to know if I’m overthinking/overreacting about this because I’m nervous she could be replacing me or if I have a genuine reason to be upset? :(