[yorkietalkjilly]

Vets always say to get any unexplained change in behavior checked out medically first thing. Just to be on the safe side, I'd get the vet to check her out and get some blood work/exam. Lethargy is one symptom of several medical conditions that aren't too pleasant. And you know, if you've had very much contact with dogs, that even a sick dog can be excited by being outside, a visitor to the home, etc. and play and eat normally for a time. But once the activity or the stimulus is past, a dog who is having medical difficulties will go usually back to his lethargic state.

Dogs are stoic by nature and routinely don't show chronic/continuing pain or nausea, feeling ill, etc., except by their behavior. So if the lethargy and disinterest in life in general returns except when incited or stimulated, get her to the vet for a good check-up. A normal dog sleeps a lot but then they are busy baiting you to play with them, notice and interact with them, patrolling the house, watching out the windows and barking at squirrels, cats and passersby, hunting out and finding their toys and playing with them or trying to get you involved with them, following you around all day and watching everything you do and generally busy being an active dog, not lying about all day.

If she checks out medically, then you can begin to consider whether or not her life is so dull and uninteresting to her and with so little enrichment that she's just given up but not until she's gotten a clean bill of health. Some highly intelligent and hyper dogs do need a lot of interaction and many varied activities with their person in order to feel important, needed and vital and may give up if they aren't getting the lifestyle their particular nature requires.