When your cat screws up her face and curls her lips back to show her upper and lower teeth this is known as Flehmening. She does this in order to get more information about a smell she has discovered by allowing the smell to register with “Jackobsen’s Organ”. This organ is located in the roof of her mouth and is also associated with appetite and sexual behaviour.
In the wild, Flehmening is quite pronounced (and is not by any means limited to cats) but domestic cats rarely use this technique, and when they do it tends to be less dramatic.