Why would a girl be nice to me & help me in my personal life just to talk down on me & leave?

I knew this one girl who I met through my siblings & she treated me well as a platonic friend for the few months we knew each other

She would help me with rides, lend me money, hang out on a platonic level

Eventually before she left to another state she apparently was talking mad crap about me to some of her friends

She never said bye or even let me know she was moving

Why would a girl be nice to me & help me in my personal life just to talk down on me & leave?
Post Opinion