Hi,
Lab studies are to be done. if I will be your treating physician, I will order  stool culture/ova, parasite analysis and, fecal smears. 
Metronidazole is reported  but can cause proctitis/anusitis as your history indicates. However, the above simple tests are still useful to rule out [as some diseases cause similar symptoms more often]
Acute cases can be self limiting and require supportive therapy [hydration, if needed anti-diarrheals, or 5-ASA enemas] antioxidants 
vitamin a,c,e can be taken for few days, I will also suggest [deep 
breathing exercise or guided 
meditation by art of living course ombw or sahaj course will take away psychological 
stress and helps increasing success rate with medication]. It should be checked by physician once and then if medicine  or surgical requirement needed can be taken.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Sunny Madar, General & Family Physician