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.
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