One of two things is usually the most common reason for stomach cramps. The first one is caused by the digestive system. Stomach cramps result when blood is redirected from the process of breaking down food to the working muscles. You have merely interrupted your digestive system which will do you no harm only cause you some discomfort. Usually you can avoid this by giving yourself plenty of time to eat before running so that it is out of the stomach and into the small intestines.

The second most common reason for stomach cramps is your training pace.  If you are running faster than your fitness capacity, you may not be getting enough oxygen to your working muscles causing a "side stitch" or stomach cramp. Running at an even pace throughout your training program will allow you to perform at an optimal level. As you improve your fitness level, so too will you improve in your training pace. However, you will also train your muscles to utilize oxygen more efficiently and reduce the likelihood of any stomach discomfort.