D.W.
Oh, I'd definitely take her to the doctor and have it evaluated.
I started getting migraines when I was 10. We never found the cause, but I needed Tylenol-3 w/codeine to help take the pain away.
With kids, it's hard to know if they're really telling the truth and understand what it means to have a head ache, stomach ache, etc. But, having gone through my own battle of cancer recently, I tend to over-react about my kids' health, and my pediatrician is completely understanding.
You may find it's a food allergy, seasonal allergies, etc. - something that requires some kind of treatment, even if Over the Counter.