Is my best friend using me?

She sounds like she's going down a bad road. Right now you need to be there for her and make sure she's okay because by the way things are going something awful is bound to happen. I dont think she's using you, but she isn't being a very good friend. Make sure you talk to her about, get it all out there. You are her best friend make sure you talk to her about how you feel if she's your friend she'll listen. Good luck sweetie!