My 2 yr old went swimminng yesterday in pond now hes runnin a fever since hes woke?

took my son swimmin to a lake yeterday. and ever since he woke up this morning hes been runnin a fever. gave him tylonal and hes napping now but the fever hasnt seem to have broke. dont kno if its from bacteria in the water or what or from the sun but he was wearing sunblock and had plenty of fluids. if anyone has any advice feel free to commentt :) thankss