Hi, I had gone through your question and understand your concerns. I see similar cases amongst young people who visit my clinic. I suggest them to do not overuse pain killers. In your case, i would like to suggest take plenty of oral re hydration salts. As you have not mentioned about the number of episodes of diarrhea, i am unable to classify the severity of diarrhea. You have mentioned about black stools. We call it as melena. As you have taken plenty of pain killers, you may have Peptic ulcer in stomach or duodenum. Bleeding from peptic ulcer reacts with normal acid secreted in our stomach and gives black colour. Thus you have black stools. If you are my patient, i would treat you by Proton pump inhibitors like pantaprazole. In further, i will refer you for gastroenterologist to rule out any peptic ulcer. Better take plenty of water with Oral rehydration salts and Oral proton pump inhibitors. I suggest you to consult a physician and get an upper GI endoscopy to rule out any peptic ulcers. Thank you.