What could be wrong with my daughter?

My daughter has been coming down with a fever every 2 - 3 weeks since late March. It lasts 1 - 3 days. In May, it was rather high. According to my thermometer, it reached 103.9. Obviously I was quite concerned. I have taken her to the doctor, but he hasn't been able to find anything wrong. They've done strep tests, but they've always come back negative. I've been told it is a virus and we have to let it run its course. She has only thrown up once. Her only complaint has been a headache and one time she did have a sore throat too. I'm concerned, b/c it keeps reoccurring. What do you think could be causing this? FYI - she is 8 yrs old. Thanks.