At 24 years old, white gums shows anemia more than COVID. Blood in the stool shoes possibly a gastrointestinal tumor, cancer, worms or parasites, swallowed foreign object that punctured the stomach or bowel. There are one or two illnesses that effect elder dogs where they are not producing enough red blood cells to carry oxygen to their muscles and internal organs. Signs are extreme fatigue, loss of interest in life, loss of appetite and general listlessness, can’t seem to get settled and totally comfortable.
Weather Covid, cancer or whatever the day you noticed the. Dog was off you should have been watching carefully. None if these illnesses get a dog to the dying stage within 24 hours. You should have taken the dog to the vet sooner.