I've never seen this, but I did some looking around - found this:
Lips licking in dogs is usually associated with stress and nervousness. As your dog is also isolated which is another sign of stress, so, there are chances that your dog is having stress.
Another reason of lip licking is excessive salvation or drooling which can be due to some specific stimuli like food, excitement etc and some pathological conditions like like gum & dental diseases, tongue injury, stomach disorders, mouth ulcers, tumors in mouth, and due to some metabolic disorders.
Lip licking in dogs can also be the part of self grooming because dogs can clean any area of the body including lips if dirty.
So, you should ease out stress of your dog or visit a vet to get advice.
My Dog Is Isolating, Licking Her Lips Constantly And Not Acting Like...