I think you should stop and take a rest or a break. If the side stitch is really painful, then don't start running until it has stopped. Or you can walk it off so it might not get any more painful when you're resting.
A side stitch, also known as flank pain or exercise-related transient abdominal pain, has several causes, but one thing is consensus: a stinging usually occurs when the exertion of physical activity is near the limit of your fitness. This physical discomfort has been much studied in recent years and can be caused by more than one reason. One of the possible reasons for pain in the side of the belly would be diaphragm fatigue when intense physical activity is being performed.
The better your fitness, the greater the degree of security not to feel the discomfort. However, by the time the side stitch appears, the best thing to do is to slow down, stop physical activity and get some rest. The annoyance will soon disappear.
