Cramps or GI. Cramps occur as a result of very vigorous breathing and are thought to be related to decreased oxygen supply to these muscles. Usually, slowing down and grabbing or massaging the affected muscle will alleviate the pain. Dehydration contributes significantly to GI disturbances. Stay hydrated.
Something different. You must be doing something different. Carefully analyze every step from waking up to completion to see what is different. Common things are diet, food intake or not, different fluids -- ie race provided vs. Your own in training. You may be flattening out more in aero. How about what you are wearing -- tightness, race belt ? Warm up exercises ? You will find it.