Hi there,
Three weeks of diarrhea is considered pathognomonic for small intestinal bacterial overgrowth (SIBO), explaining the partial benefits of metronidazole.
With a wiry pulse, you need something mobilizing and dispersing, rather than Si Miao San. Huo Xiang Zheng Qi San (Agastache Combination) might be a better option. It's a broad spectrum antimicrobial, is dispersing, and is anti-Damp. If the pulse improved in response to sedating BL 40, it's the first thing I'd try.
Otherwise, the other 'knee jerk' consideration with that pulse and history might be Bu Zhong Yi Qi Tang. If that formula were to be helpful, I'd expect a moderation of the pulse by needling SP 4 (near where many vets locate LV 3, plantar to the medialmost metatarsal).
Hope that helps. There are other options, but I suspect one of those two will help.
Steve