What does it mean when pain starts an hour before time for antibiotic 2nd dose

I'm on rifaximin 550 mg x2 / day. Both pain in my ileum and pain right below the left rib cage start an hour before it's time for the 2nd dose of the antibiotic. This pain lasts for other two hours after I take the medicine. I was prescribed the medicine coz of inflammation. I noticed too that whenever I have a bm, parts of my stool are black and not all of it. Yesterday, I had fresh bright blood but I think this could be an anal fissure. I hardly eat and when I do it's only rice pudding. I can't tell what's wrong this time or what triggered it.

Actually, no.