Recent studies point to an increased sensitivity of the sensory nerves in the intestines. Normal movements of your intestines may be perceived as cramps or other discomfort.
The intestines share nerve pathways with the brain. In many situations, when the brain reacts to something – like the sound of a dentist's drill – the intestines, or gut, pick up the same signals and react.
The majority of people will ultimately have some kind of gastrointestinal (GI) symptom when exposed to stressful situations. If your GI system is a bit too reactive, you will experience symptoms in more types of stressful situations than someone else will whose gut is not quite as reactive. What is stressful for one person may not be stressful to another, and lots of people don't even realize it when they get stressed – they just feel sick.
Finally, there is the "gate theory" of how pain is experienced. When pain originates at some point, nerve messages pass through something like a gate on their way to the brain. The wider open the gate is, the more pain that is experienced. By thinking about and focusing on the pain site, we open the gate. Plus, feelings of anger or worry or sadness can open the gate.
However, we can also help close the gate. Turning attention away from the site or feeling of pain, through relaxation or focusing on some other activity, can help close the gate and lessen or even eliminate pain.
A well-known phenomenon that demonstrates this is that of the athlete who plays a game while injured, oblivious to the pain. The athlete is completely focused on the game and does not feel pain. Then, after the game is over, the athlete turns attention to the injury and feels pain."
while the above is a question to a doctor about a kid with functional abdominal pain, its still applies.