My 5 Yr Old Has 104 Temp And Has No

My 5 yr old has 104 temp and has no other symptoms other than headache, stomach ache and he says his legs hurt. He just had this three weeks ago. He tested negative three weeks ago for strep and flu. What could it be? It come on all of a sudden and the fever it the only thing and the complaints of legs head and stomach come after the high fever