It could be stress, diet or both. But before self-diagnosing, be sure to put a call into her doctor so that any potentially serious causes may be ruled out.

Diet certainly can play a large role in the cause of headaches, as well as their prevention. There are three different areas you should investigate to identify if diet might be a factor in her afternoon headaches:

1. Dehydration: Talk to your daughter about her drinking habits during the school day. I know that when my daughter was younger, she would drink nothing all day to avoid having to use the school bathrooms. Then, she would show up every afternoon with a headache. Once I finally discovered she wasn't drinking enough liquids, we corrected the problem. Make sure that before your daughter leaves home in the morning, she has had plenty to drink. Also, pack something to drink in her lunch box. When your daughter returns home, be sure her afternoon routine includes having something to drink. You might also talk with the physical education instructor to encourage a classroom discussion of the importance of staying well-hydrated, particularly before and after exercise.
2. Afternoon low blood sugar: If it has been some time since lunch, or your daughter ever skips lunch, then low blood sugar may be the culprit. Also, a lunch that is high in refined carbohydrates and sugar may be causing a rapid rise in blood sugar, followed by a reactive drop, leaving her without fuel. The best lunch is one that will supply nutrients to get her through the afternoon. That means you need to include the slower-digesting proteins, fats and complex carbohydrates. Choose whole-grain breads, cheese, tuna, deli turkey, chicken or peanut butter. Include a piece of fruit or snack mix that includes dried fruits and nuts. Avoid highly processed foods like chips, white bread, sugary juice beverages or candy. Often kids will eat only those, leaving behind the foods that will fuel them better over time. Several smaller meals or snacks throughout the day may also help. If your daughter complains that there is no time to eat lunch, speak with the school to see what can be done to assure adequate time. Perhaps kids are skipping lunch in favor of more time in the playground. If this is the case, maybe recess can be scheduled before lunch.
3. Avoid trigger foods: There are some foods that are commonly known to trigger headaches in susceptible people. Keep a food diary and see if any of these foods may be causing headaches in your daughter: Aged cheese, chocolate, pickled or fermented foods, meats preserved with nitrates, food with MSG, nuts, citrus fruits and yogurt. If you notice a correlation, avoid those foods and see if things improve. For an adult with frequent headaches, watch out for red wine, beer and caffeine.