Nothing is as frustrating as a bad day at the day of your sport event. You carried out your training plan in detail and you gave up every bad habit for this moment. But then during your run-, bike- or rowing race all of a sudden you experience stomach ache. There is no way you can continue your pace and it feels like all the hard work was a waste of time.
If you ever experienced this, you are not the only one. Studies suggest that 30–50 % of athletes experience gastrointestinal problems like nausea, vomiting and cramping. These problems seem to occur especially during endurance events.
Fitness
Stomach cramps or side stitches are common problems during exercise. I guess everyone has experienced some form of gastrointestinal problems at least ones, for instance when running. Too often coaches, trainers or co-athletes judge these symptoms by saying you are not fit enough. Which is often not the cause.
Nutrition
It makes sense to look at your food intake when having gastrointestinal problems. An unhealthy diet can indeed be a cause. Poor nutrition timing could cause gastrointestinal problems as well. This is probably the easiest cause to change by consuming your last meal long before you start exercising (2-4 hours).
Competitive Anxiety
But there is another cause that I think is often overlooked, and that is performance pressure. In literature you will find it as (competitive) anxiety and (competition) stress. I deliberately put the word competition in parentheses, because you can experience anxiety and stress during training as well. If you notice gastrointestinal problems occur during important training sessions, on competition events, and in general when you are not sure if your abilities match the expectations, anxiety or stress might very well be the cause.
A very recent study confirms that anxiety may be a risk factor for experiencing gastrointestinal symptoms. Even though there is a correlation between stress/anxiety and gastrointestinal problems, there is not a lot of research that looks at this correlation during exercise. This study however concludes that “individuals with higher levels of anxiety, especially on the morning of a race, may be prone to gastrointestinal distress, particularly nausea, regurgitation/reflux, and cramping.”
I personally find this an important finding because of two reasons. First, if you experience gastrointestinal problems like nausea, reflux and cramping on special events, you now know you are not the only one. Second, now that you know anxiety or stress could be a cause, you can start working on the solution. This solution might have nothing to do with your previous approach which was based on fitness level or nutrition intake.
Solution
Although finding the cause does not per se mean finding the solution, it gives you a better starting position. If you have a knee injury, you go and see the physiotherapist. If you experience gastrointestinal problems caused by (competition) anxiety or stress, my personal suggestion would be to go and see a sports psychologist. There are a lot of techniques like visualization and relaxation that can help, which are far from hocus pocus. If you do not feel the urge to meet a sports psychologist (yet), you can consider a meditation app. My personal favorite is Headspace. 
Founder of Molab, Human Movement Scientist and Freelance Content Marketer.