Much of the time we don’t know is the honest answer. There are a number of diagnoses that your GP or paediatrician will try to rule out by asking you and your child lots of questions about the nature of the pain and any associated symptoms like change in bowel habit. Most children do not need blood tests or other investigations but many get them anyway. We are seeing more coeliac disease (sensitivity to gluten which is a protein found in wheat) now that we are looking harder for it but in the majority of cases, if the history is not worrying, any tests done are normal.
We term the following triad of symptoms, functional abdominal pain:
• pain that waxes and wanes
• pain that occurs with three episodes within a three-month period of time
• pain which is severe enough to affect a child’s activities (eg. school attendance)
Functional abdominal pain is difficult to manage and we know that if parents appreciate the interplay between emotional and physical health, play down the pain and address any underlying fears or worries their child may have, recovery will be faster. It is not that the child is putting it on; their brains are very good at feeling emotional pain as physical pain, partly to protect themselves and partly because they can not as yet make sense of psychological hurt. Having a tummy ache makes more sense.
http://www.keepkidshealthy.com/welcome/commonproblems/abdominal_pain.html is a sensible, reassuring American patient information leaflet which I would recommend to you.