3 year old with fever?

Yesterday my 3 year old daughter started running a fever of 103.9. We took her to the ER where the performed tests including blood and urine labs were all normal. Her white blood cell count was normal as well. They gave her tylenol which dropped the fever. After all the tests came back normal they sent her home. She had a stye on her right eyelid but the doctor said it had nothing to do with her fever. I must mention that we found a tick in her hair on the back of her head which we pulled off head and all. They did check her for lyme disease which was negative. She doesn't act sick or anything she just has a fever. No lack of appetite no lethargy just a fever that keeps hanging around 101. What could be the problem? She isn't complaining about anything hurting except her eye.