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I would worry more about Parvo, especially if the dog has not been vaccinated.
You will want to see the vet tomorrow to find out the cause. If parvo is ruled out, other causes might include parasite problem and intestinal obstructions.
Let me know if you have more questions about this.
With Parvo, you might see diarrhea with blood in it that has an awful smell, vomiting, lethargy, and neurological symptoms like falling over, stumbling, etc. You might see one of these symptoms or you might see all of them.
Keep in mind that parasites can cause the symptoms that you are seeing as well.
Your best course of action is to see the vet today.