What could cause nausea in my 11-year-old that is usually after eating?

A few things: If she is nauseated after eating it can be from a gastritis (inflammation of the stomach), viral, intestinal related, constipation, food intolerances, or other things. See your doctor but track what she is eating since it is related to food. The most common intolerance are from gluten and dairy. Try doing an elimination diet of them to see if this helps. Also consider a probiotic if is gi.