influenza
"My fever went down within a few hours and I was feeling better the next day, but weak.  After finishing the last dose of 5 days, I became extremely nauseous and my abdomen hurt so badly, I didn't think I would survive.  I didn't vomit, but stomach pain was excruciating.  It has lasted over 24 hours, so far and I can't find any advice regarding what to do about the pain."