My friend hasn't been treating me well & I think she likes another girl better, what should I do?

I've been feeling really annoyed and down lately due to my best friend not making me feel great. I think she wants to go off with this other girl but whenever I try to speak to her about it she says that she will always put me first and acts offended I would even suggest it, but that's not how she acts. It even seems like she is desperate to find new friends and that makes me feel like I'm not enough even when I try my hardest to be the best friend I can and have supported her through so much.

For example, this weekend we were going to go and watch a movie together along with another girl and while me and my other friend were talking about it and we asked her she said that this other girl had invited her over for a sleepover. We said okay and just dropped it, knowing it would take a bad turn if we said anything more. Then she said "Well, you guys can still go if you want" in this really sarcastic and challenging tone and I just didn't reply.

It ended up turning into this huge thing and she avoided us and cried and it was really dramatic. And just like what always happens, I end up giving in and comforting her and telling her what I really feel and why I did what I did and then she guilt trips me. It literally happens every argument.

Does it sound like she wants to leave me, what is your take on this?