Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear about your pup. She sounds very sick. Dark/black liquid or tarry stools usually indicate digested blood in the stool.
Diarrhea can be caused by a primary viral, bacterial or protozoan infection, or eating something that irritates the gastrointestinal tract, inflammatory bowel disease or a severe case of parasites, an obstruction or twisting of the bowel leading to intestinal wall damage, internal organ failure or cancer.
The danger with blood in the stool is that means the intestine is raw and a raw intestine more easily allows bacteria into the abdominal cavity itself and can lead to peritonitis. The darker color of the blood in her stool indicates that the blood is in her stool is from farther forward in the intestinal tract.
Can you check her temperature for me? Normal dog temp is 102F. A fever is >103.5F.
Can you also check her mucous membrane (gum) color? They should be bubblegum pink and if you press on them they should blanche then return to pink in less than 2 seconds. They should also be moist not sticky. If they are sticky that means she is dehydrated.
If her color is fine and she doesn't have a fever or a subnormal temperature (less than 99F) then she is more stable than a dog with a high fever or a low temperature, but I still would recommend an emergency examination tonight if her symptoms have been going on for more than 24-48 hours and/or she seems lethargic.
If she has a fever, her mucous membranes aren't pink, and when you press on her abdomen it seems very tense or painful I would be more even concerned. These are signs of a blockage, pancreatitis (inflammation of her pancreas causing increased enzyme production, usually related to a high fat meal) or possibly a viral infection. These are emergency situations and she needs to be seen immediately.
If she cannot be examined tonight do not try to feed her any more. Fast her for at least 12 hours. This may stop gut spasms and help restore normal motility. Right now hydration is important because of the diarrhea and vomiting so make sure she's drinking well. It's a good idea to offer pedialyte too in a few hours to replace electrolytes. If she won't drink on her own you will need to dribble it in with a syringe (2 to 3 teaspoons at a time). If this goes well (no vomiting) then in 12 hours you can try a bland diet. If she continues to vomit even without food then she needs to see her veterinarian for more diagnostics (bloodwork and radiographs and/or an ultrasound of her abdomen)
To make a bland diet boil hamburger (or chicken) chunks, drain off all fat, and mix 50:50 with plain boiled white rice. Feed 2 tablespoons initially. If she handles that well then feed her 1/4 cup of the mixture a couple hours later. If she handles that fine then feed her 4 meals of this mixture daily until she is feeling herself (usually in 2 to 3 days) and the diarrhea is resolved. Once she is eating well for a few days with no diarrhea start mixing in her regular food and slowly (over 7 to 10 days) convert her back to her regular diet by adding more of her regular food and less of the bland diet each day. If she starts vomiting, runs a fever, has a painful abdomen or stops eating she needs a veterinary examination.
You can also give her Kao-pectate to coat her intestinal tract and absorb bacterial toxins at 1ml per pound every 6 hours or 1 tablespoon per 15 pounds of dog every 6 hours. This should help stop the diarrhea and it is very safe to use for 4 to 5 days if need be.
If she continues vomiting , has increasing amounts of frank blood or dark tarry stools (suggesting digested blood), has a painful/tense abdomen or a fever (anything greater than 103.5F) or below normal temperature then she needs to see her veterinarian as soon as possible as this suggests more serious disease that can't be treated at home.
Either way I would suggest submitting a fresh stool sample to her veterinarian to check for gastrointestinal parasites.
Her limp may be completely unrelated, but in some instances of clotting defects (severe liver disease, primary clotting deficiencies like Von Willebrands Disease) or tick related or immunosuppressive viruses we can see bleeding into the joint too, thus the limp.
Please reply back to this post with any further questions.