My name is***** and I am sorry to hear your grand-dog has diarrhea. There are many things that can cause diarrhea in young dogs so she will likely need to see her regular veterinarian for diagnostics and treatments.
The following is a list of possible causes of diarrhea in young dogs, in no particular order-
-dietary indiscretion- eating something she shouldn't that caused severe irritation causing diarrhea
-intestinal parasites (hookworms, roundworms, giardia, coccidia). Your vet will run a fecal exam and prescribe dewormer to treat any parasites that are found.
-food allergy- uncommon but not unheard of in young dogs. If the fecal is negative and the diarrhea persists in spite of symptomatic treatments, a food trial with a low residue diet such as Royal Canin GI Low Fat is warranted.
For treatment over the counter, I would only recommend a probiotic such as Fortiflora. Peptobismol is not effective for dogs and can cause other problems such as stomach irritation.
Your veterinarian will likely recommend an antibiotic such as metronidazole, a dewormer, and a probiotic.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001