Howling when you're not in the room is a sign of separation anxiety. This may or may not be related to the lethargy. One explanation could be if the dog feels unwell and you are a source of comfort, the dog may howl when you aren't there. If this is the case the dog needs a vet checkup to find out why the dog feels unwell. In the case of separation anxiety, be sure only to re-enter the room when the dog is quiet, so you reward the good behavior rather than the howling.