How old is she?
how long is she like this?
is she eating?
In a cat this young this can be a symptom or a broken tooth, a sore in the mouth, licking a dsagreeable sunstance, soemthing stuck in the mouth or nausea, pain etc.
Try to examine the entire inside of her mouth.
See if she will drink milk or tuna water.
Put a drop of pancake syrup in her mouth to raise her bood sugar in case she is hypoglycemic
As long as she is eating this is not an emergency but she should be seen in the morning if it continues.
Please let me know how she is doing. I will akays be here for you.
Sincerest best wishes,