Why did this girl call me and hung up on me?

This girl I'm seeing cqalled me today and I answered and I said 'whos this' then she said her name then I said 'whats up' then she hung up the phone and texed me soon after saying 'whats good' why do u think she acted like this.Sometimes shell call me and when I answer she hangs up, and texts me but when I asked her the first time it happened she goes 'oh I didn't know u answered'