Fever but not acting sick

My 3 year old daughter has a fever of 39.7 but not other symptoms. I gave her Advil and the fever has gone down to 37.6. I’m hoping it doesn’t come back. She’s in good spirits laughing playing, eating and drink just fine. She’s just super hot and looks extremely tired. I’m praying she’s better by morning. Any idea what could cause this. Has this happened to anyone else’s children. She literally has no other sign of illness other than the fever.