There are different types of lip licking and there are different reasons for them.
She might like your petting so much and becomes so relaxed that she actually starts drooling. Once you stop petting that sweet spot, she notices her drooling and licks her lips.
She might also have been taught that she shouldn't lick people. Some dog owners allow their pets to lick their hands or faces, others don't. Since licking is an ingrained instinct, the dog licks her lips instead of your face.
If she licks the side of the whole mouth all the way to the cheek, it's a sign of anxiety. This is often accompanied by loud yawning (which has nothing to do with her being tired). You might be petting spot she actually doesn't like you to pet. Or she might ponder how she could get you to per her sweet spot again.
Another way of lip licking is right in the center of the mouth. The tip of the tongue flicks out and rapidly licks just the upper lip. This is a sign that the dog is uncomfortable and wants you to stop doing whatever you're doing now. It could be accompanied by some growling and is a sign that the dog strongly disagrees with what you're doing.