<human>: I woke up Thursday, 6 days ago and had to run to the bathroom with awful diarrhea that last half the day. My stomach is sensitive to stress and I had an exam that day, but I have never had more than one loose stool from it and never 7+ stools like that morning. Saw my college doctor who told me it is likely stomach flu and gave me anti-diarrheal which helped that day. The next day I felt better so tried to start incorporating normal food back into my diet, but it took a turn for the worse and I spent the next two days going between extreme amounts of gas buildup to painful diarrhea and stomach pain. I went back to the doctor for bloodwork on Monday and the results came back saying there was nothing elevated to even show my immune system fighting anything (or so I understood). Monday morning was by far my worst morning in the illness so far. What does this mean - does it mean that this isn't viral or bacterial? What can this be, and why do small lapses in what I eat cause such bad consequences for days (ate a sandwich with mayo Saturday night)?

Could you distill the essence of the text into 1-3 sentences?
<bot>: Stomach mystery. I've had bad symptoms of what I thought is the stomach flu for 6 days with major ups and downs, but my blood work doesn't indicate anything outside normal range. What can this be?