Hi. Thanks for your query. Noted your history of being with your friend last Sunday. She has bouts of diarrhea which she thinks got it from her co-worker who has nausea. Thinking of Viral/flu casing stomach upset. You developed black stools, pain in abdomen, upper rib pains and nausea. This can be a stomach virus that can cause such symptoms if yo are minimally affected. I would advise you the following: Get the stool checked and see if there is an active infection and if present get a course of an antibiotic and supportive medicines like PPI and so on. The black stools indicate bleeding somewhere in the GI tract, most probably in the stomach as ulcer and you can get this confirmed by getting upper GI endoscopy and colonoscopy. Also get the blood and urine checked. Further treatment will depend upon the reports.