That is too bad about your vet not doing a fecal test. Any other vets in the area?
I am not familiar with the condition you described. "Explosive" diarrhea is very typical in humans and dogs who have Giardia and usually treated with Metronidazole. Also when taking antibiotics it is good to take probiotics. Some antibiotic medications have a side effect of diarrhea. Putting your dog on a bland diet is very helpful for a few days, relax the GI system.