The most likely cause for these symptoms in this girl would be an infectious gastroenteritis and if there is fresh blood in her feces this indicates a large bowel (colon) diarrhea, which can commonly be caused by salmonella or campylobacter. Definitely be sure to take a sample from her in an icecream container or something similar to your Vet tomorrow morning, so that they can send it to the lab for a fecal culture so that we know what is going on here. Diarrhea like this can also occur with intestinal worms (so be sure to have your girl wormed if she is overdue for this). A culture would also help rule out giardia which is also a common cause of such a diarrhea and needs quite a long treatment of metronidazole. Hopefully this is just a bit of colitis that settles down quickly. In the mean time, be sure to offer plenty of fresh water and even think about making her some cooked, boneless, skinless chicken breast and rice.
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Kind Regards,
Dr M D Edwards
Dog Veterinarian
Veterinarian - BVSc (DVM equivalent) - Webmaster & Head Veterinarian for www. onlinepetdr.com