My sisters been sick for a week

Last weekend she had a really high fever - 103 I think. She went to the doctor sunday and he prescribed antibiotics. All week her fever hasnt gone below 100. - 101 one day and then up to 102 the next. She's called her Dr almost everyday sayng that her temp still isnt normal and he's told her to keep on the antibiotics. I told her to find a new Dr, that this one doesnt seem to be taking her seriously.

What could this be? SHes had no other symptoms other than a high fever FOR A WEEK