Your crush said hi because she was most likely just trying to be polite, being that the both of you haven’t spoken for a while.
However, when she saw that you were smiling at her, she most likely got worried that you still like her.
She turned around really fast so that she wouldn’t put herself in an awkward position where you start misinterpreting her politeness as a sign of romantic interest.
She rejected you before for a reason. That reason most likely hasn’t changed.