I have a headache after having severe diarrhea yesterday. is it ok to take advil (ibuprofen) or will it upset my system?

Dehydration: Try drinking a lot of water and taking some soup or broth.. that will help resolve your headache because most likely it's due to dehydration. Sometimes a virus can cause a headache and diarrheal type illness but I would try REHYDRATING first. I think that will get you some relief! Advil (ibuprofen) or Motrin could be damaging to the kidneys in a dehydrated state-so that's another reason I would want hydration