Is it normal to have a headache after having fever (38.3c) following stomach flu? fever has gone down but now has a dull sore headache. not pregnant

Yes: Yes it could a sign of dehydration so drink plenty of water and juices or it could a reaction to a low grade fever, take Tylenol (acetaminophen) . Or could be a pressure from sinus congestion take two Benadryl and a Tylenol (acetaminophen) at bed time and follow it with non drowsy allergy sinus medication like Zyrtec, Allegra, or Claritin . Not better make sure to check the BP and make sure it is not elevated and see a doctor