She most likely has a kidney
infection, which can come from the bloodstream as in flu, or is a result of a urinary tract infection from pool water, or possibly losing her balance and bumping her back where the kidneys are. I think, tho, since she had a fever, it's most likely a kidney infection. The docs at the ER would have noticed, by the way, if she had a bruising on her back, so I do not think she has ruptured her kidneys or anything.