Friend being spiteful?

Right basically, recently my best friend has started being slightly spiteful towards me? Half the time i know she's just joking, but other times she just takes it too far and her being like this has just made it awkward between us. I have no idea why she is acting this way. For example the other day she was casually just like 'is that a new top?' and pulled a disgusting face. or she'll just come out with snappy lines like 'i dont care', I KNOW she's joking but its just a bit like, taking it too far. Even more frustrating thing is, the next minute she'll be nice to me again. aaahhhhh why is she doing this?