Then she isn't your best friend and you should probably find a new one. Anyone who is willing to slander your name without any remorse doesn't deserve your friendship. There is a reason behind her actions:
Being jealous, not really liking you, liking you *too* much, or even wanting your boyfriend.
You may not have many friends, and it could be hard for you to do this, but you need to go to her and tell her to straighten out or you two can't work. If you can't trust who is closest to you, then who can you trust?