I think you've nailed sundowning there. If Lucy sleeps well in her crate, you might consider putting her in for an enforced nap a few times a day if she doesn't seem to nap on her own. Whether they seemed tired or not I put my pups in for a nap mid-morning and mid-to-late afternoon. Just like a cranky child, they often seemed more pleasant to be around if they weren't overtired.