When i was standing in the shade after warm ups at swim waiting for instructions with my team i suddenly felt weird my vision was going black. and my stomach started to really hurt i had to grab on to a pole that was near me for balance why?

Could have been dehydration or the effect of sunlight if it was sunny or the effect of chlorine if you had been swimming without goggles( or inadequate goggles) or even reflux or effects of blood distribution if you ate right before swimming .Even in a pool keep hydrated If you have a history of migraine could be migraine variant. https://jamanetwork.com/journals/jama/article-abstract/386706

I think you had a vagus nerve reflex reaction to a transient gastrointestinal spasm of some sort. The vagus nerve reaction slowed the heart rate and decreased your blood pressure causing presyncopal lightheadedness and/or blanking of your vision. Were you able to subsequently partake of the meet? If so, your GI spasm thankfully wasn't a serious issue.