This list is endless on what could be wrong. You need to take your dog to the vet and get is properly diagnosed and treated. The vet will run the proper tests and tell you exactly what the problem is and how to treat it. Please get your dog to the vet as soon as you possibly can, because the longer you let it go, the worse your dog will get and the harder the problem will be to treat. Acting lifeless one day and not the next and not eating, are signals that something is very wrong and you really need to find out what it is. A vet is the only one who can diagnose and treat your dog.I hope it's nothing serious and that your baby is back to normal soon! 
