E.P.
My son is 9 years old and this happened to him at the beginning of the school year. Had a fever, then went away; next day, went up to 102; then went away. Got as high as 103 and he usually doesn't run high fevers. Had a headache and tiredness, some of the time, but no other symptoms. I just thought the headache was from the fever. No sniffling/cold symptoms. Did not even suspect the sinus infection diagnosis I got, three days later, with lots of fluid in his ears. My son has some environmental allergies but wasn't exhibiting those symptoms. Good luck. Hope this helps.