What would it be caused by? Is it serious or fatal:confused:

It could be any of a number of things. Unfortunately, one of them is parvo which quickly kills may young puppies. Unless your puppy is at least 4-5 months old and had a series of shots, I would find emergency care. Not every bloody diarrhea is parvo, but do a search here on parvo and see how many puppies with symptoms like yours died. If it is parvo, its only hope is prompt IV fluids.

Lethargy is another bad sign, perhaps good cause to rush even an older puppy to the vet.

Emergency care is expensive. I hate to put you to the expense unnecessarily, but I don't want to read another post about a puppy that died.