Vomitting immediately after pushing your body to its limit is fairly normal.
Someone correct me if I’m wrong, I believe the reason is because your body redirects blood/energy to the most important parts when your body is under strain.
Edit: Please see the correction provided by EpicFailGuy below.
So, if there’s too much food in your stomache and you’re pushing your body to its absolute limit, the stomache becomes less important as to whether it needs to continue working well. Blood is directed away from the stomache and you will vomit.
This is, at least, what happens when someone goes through shock, I’m assuming something similar is at play when pushing your body to its limit.
If swimmers end up sick or contracting some diseases, that’s when we should worry. But we won’t see those effects as immediately as the end of a race.