could be a case of a viral infection. There's a high possbilty that the virus is still present in HS body. As the virus has a repeat cycle , while it's replicating in the host cells, this could take up a few hour to day depending on the virus type. Afterwhich the cell will burst and released all the virus into the host body. This is the time when the T-cell receptors will detect the antigen in the body and activate the macrophage to kill off the virus. However , if this is a new type of virus your body has not encountered before, it will take some time for the anitbodies to be develop and identify the virus. The reason for fever is basically self-invoke. As most bacteria and virus cannot survive certain temperature, the body iimmune system will auto raise the body temperture in a attempt to kill off the virus. That explain the fever.
Just take some panadol to bring the temperature down and have ample rest. This is to prevent the body from overworking so that it can concentrate on fighting off the virus.