Diarrhea should clear up in a few days. Once it is going on for 3 weeks, you need to do some testing to find out what is causing it and then treat that condition. This is significant because it s been so long and you ve been experiencing pain with it. You could be having colitis, diverticulitis, or some type of bacterial or parasitic infection. You haven t responded to Pepto Bismol, which often works well for diarrhea, so it s time to see a doctor and find out exactly what is going on. Hope this helps.