My daughter (9 yrs old) has had nausea for the past 7 days or so. She has no fever, no headaches, flu symptoms or any other symptoms. What could it be?

Many things. Anytime a child has symptoms for that long, I think getting a good history and exam is important. Not having other symptoms is good, but nausea can have many causes. It can be from infection, medication, allergy, anxiety, reflux disease, gluten/lactose issues and so on. Best to sit down and explore thoroughly with a provider you trust.