Side stitches!!

Hello! Recently I have been getting terrible cramping directly under my right rib during, and only during my races. I try deep breathing to push through it but sometimes it gets so bad I have to stop pedaling completely! I've done a little research and found that this may be something called side stitches, and no one really knows specifically why it happens. Some think it's due to dehydration, others say it's due to posture, but no one really seems credible that I have read. I'm basically wondering if anyone else has experienced side stitches and been able to work through it. I'm hoping it has more to due with lack of fitness rather than something that is more longterm and serious. The strange thing is that during some of my toughest road races I didn't get them, but it will pop up during a short crit race, super frustrating!! Any info or tips would be greatly appreciated! -Dan