It could be something as common as pollen. Allergies are so tough to determine in dogs. My Cooper has recently been licking his mouth and has a runny nose. I've come to realize that it really only happens at night in the bedroom and have a feeling it could be something like dust mites
Is there any one time that you see her licking more than others?
I don't have that much allergy experience but I bet some other members can advise you.