Although you don't say over what time period the stool problems occurred, the most likely causes are intestinal parasites, eating something that upset her digestive system, and other problems that would require a vet to do blood testing, etc, to determine diagnosis. Excitement diarrhea is common in dogs, but usually lasts for only an extremely short time, rarely more than a day. A sudden dietary change can cause diarrhea for a few days, although changing to a food that the dog doesn't handle as well as what it was fed before the change, can cause persistent diarrhea. Some breeds such as German Shepherd Dogs don't do well when fed dog foods with soy as a main ingredient. Soy based foods can cause flatulence, diarrhea, and much more stool than the higher quality foods cause. How solid or loose a stool is, depends on how frequently the dog defecates. The longer the feces remain in the colon, the more water the body extracts from them. Diarrhea is caused when something is making the food move too rapidly through the digestive tract and the body has less time to extract water from the waste before elimination. Constipation results when stools sit too long in the colon before being evacuated, thus too much water is extracted by the body and the stool becomes more dry and hard. Green stools are caused by food moving through the body too quickly, so that bile isn't reabsorbed back into the body. The faster the food moves through the body, the more you're likely to see the green color of bile in them. Bile is also the reason why its common to see greenish or greenish yellow coloration in vomit when an animal whose stomach is empty, vomits. If a dog food has been dyed to look more attractive to humans, the dyes can change the color of the stools to a color similar to that of the food that was dyed. There is less dye in animal foods than there used to be, but when foods used to be heavily dyed, it was common to see yellow and orangish colors in stools and cat foods often were dyed reddish and thus cats would have reddish orange stools. The new dog might have possibly brought parasites onto the property. One protozoan parasite that often causes symptoms similar to your description, as well as being able to cause a dog to be nauseous, unwilling to eat, and can cause a dog to lose condition, is GIARDIA. Giardia exists everywhere in the environment, most mature healthy adult dogs become immune to it until they are stressed. Young and old dogs are more susceptible to diseases and parasites because their immune systems are weaker than that of healthy adults. Old dogs can develop problems related to aging, that can cause changes in the body and age related problems can be the source of the symptoms you describe. Have your dog checked for intestinal parasites and then if necessary, let the vet do bloodwork. Bloodwork results will tell if failure of organs such as liver failure, cancer, or some other such problem, is an underlying cause of the dog's stool problem. Stools are a good indicator of an animal's general health, a normal colored, normally solid stool and normal defecation without straining a lot is a sign that the animal is in good health and not overly stressed. When the stool changes from the normal, its an indicator of something being wrong. That "something" may be only excitement for example, like excitement of a dog who goes on a car ride the first few times, or going to a dog show (which is why wise obedience competitors always make sure that their dog doesn't have to eliminate when they get ready to go before the judge, elimination in the ring is an automatic failure in obedience competitions.) If the dog's problem is due to parasites, the stools should become normal after the parasites are eliminated. A dog with giardia usually shows a lot of improvement in symptoms within 24 hours after being given medications to treat giardia. Even if the vet is unable to find a quick diagnosis for the problem, there are medications the vet can give you that will help with the problem while the vet seeks diagnosis and a long-term solution.