It's been maybe 2 days but I became best Bffs with this other girl that my best friends doesn't like and now she won't talk to me. She is starting to tell people that I'm mean and rude. What do I do? What did I do wrong?

Don't show her that it bothers you and wait it out she should talk to you after a while and after 2 weeks try to talk to her again. And you did nothing wrong people are just difficult sometimes

No one should be jealous of a friend being friends with someone else too.
So you need to choose. Only you can decide. You might say something like 'I don't want to lose you, but don't feel that it's fair to tell me who else I can or can't like too.' That tells her she has to make a decision whether to lose you or not.

You have done nothing wrong, but she is involved in bad behavior because of her jealousy. Real friends will know this, and the fake ones are to be ignored.

So you are hanging out wif this other person so she is feeling like the third wheel which is why she is spreading it around saying you are mean so talk to her and ask her if she wanted to hang out wif you