Why is she doing this and what should I do?

If she turns you down again, just don't ask her out anymore. Keep texting and being friendly towards her, but don't invite her anywhere. If she has any interest at all this will really bug her and she'll probably start to like you more. Wait maybe a week and a half or two weeks and then try again. If she tries getting out of it again, it's obviously because she's not into you.