"What happened was that it was hard to breathe and the dress was very tight and weighed a lot," Rivera told The Associated Press in a recent interview.
Rivera briefly fainted after posing for pictures and taking questions at a post-pageant news conference Sunday night. She was given liquids and quickly recovered.
Her fainting spell was also caused by the heat, the lights and the emotional rush of winning -- not from a lack of food, as some had speculated, she said.
"I eat very well," Rivera told the AP. "My nutritionist ordered me to eat six times a day."
The 18-year-old communications student and aspiring actress has been studying English for seven months. She spoke entirely in Spanish at the Miss Universe ceremony, and during the interview.
"Spanish is an extremely romantic language that can make any person fall in love with you," she said. "And that's what I wanted to create with the jury, to be able to make them fall in love through my native language."
Rivera, the fifth winner from Puerto Rico in the pageant's history, described herself as a "boricua" -- slang for Puerto Rican -- who doesn't favor statehood or independence for her Caribbean island home, which has been a U.S. Commonwealth since 1952.
"I don't go for either stance," said Rivera, who nonetheless defends Puerto Rico's separate status from the U.S. in the Miss Universe competition.
"If we were to compete as a normal state of the United States you would lose the charm of the beauty of Puerto Rico," she said. "Puerto Rican beauty ... is totally different from American beauty."
Her first planned stop after winning the Miss Universe crown was Japan. Then it's off to Indonesia and South Africa, visits that should be confirmed before September, a representative said.