This could be due to viral illness where there will be no localizing focus of infection but fever
and headache will be there with soreness of mouth. Continue tylenol
in prescribed dosage every 6th hourly if there is fever and maintain hydration and nutrition by giving plenty of oral fluids and soft diet
and if still fever continues consult your Pediatrician
to investigate for the cause of fever with complete blood picture, malarial parasite and with complete urine examinations like basic evaluation depending on your doctor advise.