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For diarrhea/ colitis, I usually have owners add fiber to the diet to help firm it up. To do this, you can give 1-2 tablespoons of canned pumpkin with her meals. This is often enough to get the diarrhea under control over 2-3 days.
You could also try a dose of Loperamide (Imodium). You can give 1 tablet (2 mg) every 12-24 hours for 1-2 days.
If this is not helping, then it's probably infectious and some Metronidazole from your vet would be needed.
I hope this helps, let me know if you have any other questions.