Like others have mentioned, it can be a sign of nausea or indigestion. It can also be seizure activity. Or simply, food stuck on the roof of her mouth or teeth...I call it peanut butter action or lizard tongue loll.
It can also be behavioral:
knowing Your Dog’s Body Language STRONGDOGZ? DOG KENNELS TONGUE FLICKING
If your dog repeatedly flicks his tongue up to lick his nose, he’s uneasy. He may be assessing a new situation or wondering if he should approach a stranger. Or he may be concentrating hard, trying to master a new obedience command. While a tongue-flicking dog may be friendly, don’t approach a strange tongue flicker the dog is obviously tense. Tongue flicking sometimes precedes biting.