The pre/during event nerves can be a cause. I couldn't understand why on my first couple of long distance events I felt sick. I don't think of myself as being particularly worrying sort, but when I look back, difficulties at work caused quite severe gastric problems, although it took me quite some time to realise this. Once I made the connection between stress and feelings of nausea the problem seemed to resolve itself.
Another factor could be that feeling sick, for whatever reason, can lead to the vicious circle of not taking enough fluid+nutrition which actually makes you feel sicker so even less inclined to eat and rehydrate properly.
All the best and hope you find a cure.