What do i say to her when she was the one who said we just arent bestmates anymore?

A good friend would understand that you had a previous arrangement with you b/f - it could just as easily been her going out with a b/f and you would have to understand - that's friendship, sometimes you see loads of each other, sometime less, but the friendship remains the same. If you want to still be best friends with her, talk to her calmly and without arguing, tell her you want to be her friend just as always but that you have to make time for your b/f - be kind but firm and friendly.