Persistent licking could suggest:
1. Oral foreign body. So, something stuck between the teeth or in the roof of the mouth. It can be difficult to fully evaluate, so for that reason alone, a vet visit is well worth it.
2. Nausea. Many pets with stomach upset will begin to lick a lot.
3. A petite mal seizure. Seizures can sometimes manifest with awkward licking motions.
If she has been eating just fine, than nausea is less likely.
Something stuck in the mouth or teeth is still a very real possible explanation.
Just in case of nausea, it wouldn't hurt to give some Pepcid A.C. antacid. A dog around 10 pounds gets 1/4 tablet of a 10mg Pepcid. A 20 pound dog 1/2 tablet. A 30-40 pound tablet just give the entire tablet.
But, if this has persisted for several hours, I definitely vote for the ER trip to get a good examination performed to figure this out.