I agree with ennay; whenever I get a stitch it's my breathing. Just try and pay attention to it, like she suggested. I just focus on breathing slowly and deeply and slow my pace a little until it passes. When I first started running I found it was my breathing that always did me in before my legs. Once you get that under control your runs will be a lot more enjoyable.
Also, try focusing on keeping your upper body relaxed as much as possible. You might be tensing up one side without realizing it.