best friends

Okay so my best friend started lying and talking shit on me today saying I was talking to all these guys and playing everyone and stuff. What do I do? Leave her or try to fix things. She was just at my house acting like nothing was wrong. I'm so pissed! Help!!!!!!! FYI I just want to be back with my ex that she hates which when I go see him doesn't cut into our time and she doesn't know about except for when I tell her occasionally that I talk to her which I rarely do because she talks shit on me...