Williams had been using a substance which was "clearly outside the medical code," St. Kitts Olympic committee vice-president Dennis Knight told The Associated Press on Sunday.
Knight said Williams had not tested positive, but the team acted after consulting with the World Anti-Doping Agency "to find out about the product."
"In discussions with our team management, she volunteered to them that she had been using a particular substance which, when we did our own investigations, we considered to be outside the accepted medical code," Knight said.
Williams told team officials about using the substance — which the team has not disclosed — in a pre-Olympics training camp.
"It was a matter of the management of the team doing their due diligence," Knight said.
The 22-year-old Williams had qualified for the 100 and 200 metres, and gave samples for anti-doping tests at national Olympic trials last month.
"It was not based on any positive drug test. She turned up a clean test," Knight said.
Williams marched at the Olympic opening ceremony on Friday as the only woman in a seven-member team from the Caribbean islands, who are all track sprinters. The best known is five-time Olympian Kim Collins.
Knight said St. Kitts team officials sought expert advice in London before acting.
"We wanted to consult with the anti-doping fraternity," he said. "We are a very tiny country with limited knowledge of these things."