Rule out Spondy's. It could be something as simple as sore muscles or a strain, but if it doesn't resolve within a week or so, i would have her checked out. There's a back condition called a spondylolysis which is most common in teenagers. It's a sort of stress fracture. It can be diagnosed with an oblique x-ray. Your doctor will know what to do.
Many possibilities. There could be many causes of low back pain for girls. This could be simple problems such as constipation or gas pain or simple muscle strains or musculoskeletal pain. It could be more significant such as a UTI or kidney infection, especially with fever or vomiting. It could be signs of an infection, even an std. See your doc and get evaluated. Bring a list of symptoms and even a calender to sho.