Around 2,000 people took part in the events in Sunderland, northeast England, last weekend, which included swimming off the city's Roker beach.
The UK Health Security Agency said this weekend it was investigating the possible cause of the outbreak and will be asking those affected to complete a questionnaire and provide a sample for testing.
Routine tests carried out by the Environment Agency at Roker in late July, days before the competition, found high levels of E. coli bacteria in the water.
But British Triathlon, a governing body for UK triathlons, said these tests were taken outside of the body of water used for the competition and the results were published after the weekend's events.
It said its own tests met the required standard.
One of the participants, Australian triathlete Jake Birtwhistle, said on Instagram that he had been "feeling pretty rubbish since the race".
"The swim should have been cancelled," he added, posting a picture of a graph with the results of the water tests from Roker.
The competition was held in an area of the coastline that has been at the heart of a long-running dispute between campaigners and the government over sewage discharges, the Guardian newspaper reported.
Local water company Northumbrian Water insisted there have been no discharges "that might negatively impact water quality" at Roker since 2021.
The Environment Agency said "temporary dips" in water quality can be caused by various factors "including heavy rain".
In Paris, a pre-Olympic open water swimming test event in the Seine was cancelled this weekend due to pollution following recent heavy rainfall.
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