Parvovirus is one possibility here. It would explain the symptoms like diarrhea and lethargy, and usually affects younger dogs.
Parvovirus is a bacterial infection that is transmitted through feces and soil, and can be fatal if not treated.
Things you will notice are:
loss of appetite
vomiting (usually clear and foamy)
diarrhea (sometimes with blood in the stool)
Stronger smelling poop
However, lethargy and diarrhea are also common symptoms of many infections, so there's nothing to say for sure that parvo is the case here.
What I can tell you for sure is that your dog needs a vet as soon as possible!