Hello and Welcome to ‘Ask A Doctor’ service. I have reviewed your query and here is my advice. I have reviewed your query and here is my answer. The black diarrhea with burning urination is due to side effect of Bactrim DS tablet. Immediately stop its further intake, drink plenty of liquids, take 200 gm of fresh curd morning and evening for probiotic support and follow up online about the progress. There is no need of ER visit unless there are more than 5-6 frequency of stool per day. Hope I have answered your query. Let me know if I can assist you further.