One reason is Buyer's Remorse, or that it was only meant to be an ONS.
She might also act uninterested to stop herself from developing feelings and turning it into a relationship, as it inconveniences her at the time.
I've had a couple of girls like that, who were going to go overseas for work/study for a year or so, and didn't want it to develop further, so they appeared cold towards me. However, when they came back home, they changed their attitude completely - getting in touch, checking whether I have a GF, if I'm still into them, acting jealous when I pay attention to other girls etc - because now they are in the right position to start a relationship. Except that I'm not always there for their convenience.