K.M.
Ok, think of it like this: the white blood cells are the little warriors against infection. She's had a fever for several days and so they've been fighting the good fight and have lost some of their members. Because she's still fighting the infection, there hasn't been enough down time in her immune system to be able to bounce those numbers back up to normal. Thats just how the body works. I'm assuming that they did a CBC and differential, which would have shown them if anything was seriously out of whack. As long as she shows signs of improvement in the next few days, I wouldn't worry about it. However, if she is still feverish after a week, you might want to take her back in.
Lots of fluids, lots of rest, and lots of good thoughts!
K.