What Causes Fever,nausea And Headache?

My 9 years old daughter is having slight fever, nausea and headache for 4 days after she enjoyed exiting outing ( air conditioned exhibition hall, without jacket)
and played in the swimming pool. first day we took her to clinic after paracetamol and nausea medicine she was ok . following day she got the same problem. what could be the reason?