One interesting source is Heribert Watzke's Ted Talk: http://www.ted.com/talks/heribert_watzke_the_brain_in_your_gut.html

You could also look at http://www.psyking.net/id36.htm

The stomach pain from mental distress is quite a common phenomenon, due to the enteric nervous system (also referred to as "the brain in the gut"). We have an amazing number of neurons in our digestive system-- roughly the size of a cat's brain. Strong emotional responses (like fear, anger, or disgust) are transmitted from the brain in the head to the brain in the gut, often resulting in pain or other discomfort.