What Causes Dark Coloured Stools While On Amoxicillin And Prednisone?

I have dark stool for 2 days . I had sinusitis and doctor gave me amoxacilin an prednisone. I took in morning and ate. I felt like I had sour stomach. I took pep bismuth tabs . Stool was dark. Then next day had few beers and same thing so I took tums. Cuz I heard pepobismo causes dark stool. Is beer going to cause internal bleeding . Or do u think it just from antibiotics or prednisone or pepmo.