do you mean she has a fever today and for the last three weeks she has been feeling ill. i would say it was probably morning/all day sickness for the last few weeks and she might just have the flu now. if she has a fever for longer than 48 hours call the doctor. do your best to cool her down but keep her comfortable of course. hope she is feeling better soon.
if she had had a fever for 3 weeks she needs to all the Dr ASAP