Take her to the doctor. There are all sorts of funguses that thrive in lakes. In Seattle, there is a fungus associated with Green Lake that is so well known that the rash it produces is called the Green Lake Itch. She probably has a fungus infection that settled in the tender tissues of her vagina. You could try an OTC product, but if it is not appropriate for the particular fungus, you would have wasted time. So get her to the doc and get the area swabbed, so he or she can prescribe the appropriate medicine.
WHAT SHOULD I DO IF MY 7 YEAR OLD WENT SWIMMING AT LAKE AND HER VAGINA AREA BURNS