Hi,
This could be caused by a number of things including blockages or a parasite infection. Any time your dog is lethargic it is best for him to see his vet. With the behavior changes I would get him in. Most vets have a emergency contact number that you can reach them and ask about a Saturday appointment.
Make sure you are watching him for any new symptoms including blood in the vomit or feces. (if these symptoms arise a trip to the vet will need to be made soon).
Try feeding him some boiled chicken and rice to settle his stomach, also give him some pedialyte to help prevent dehydration.
Walter
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