Her simple, honest explanation immediately went viral, sparking an avalanche of support for the bronze medallist on Chinese social media.
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“The state spends a lot of money on her - and she’s talking about her period? Good joke, but why do you have to play the idiot in front of the whole world?” wrote one critic.
“Your mothers, aunts, grandmothers all had periods - it’s natural to talk about this in 2016,” retorted another Weibo-user.
Fu has become the biggest Chinese star of Rio, with her over-the-top facial expressions inspiring emoticons and going viral online.
She now has more than six million followers to her Weibo account and been hailed by Chinese state media for showing another side to the more typical robotic athlete stereotype.
As she flew back into Beijing airport Tuesday, she was greeted by crowds of fans clamouring for a selfie.
Fu has always had an enthusiastic pool-side manner - a video from the World Championships in Kazan in 2015 shows her grinning with joy and shaking her head on the podium, alongside her more serious teammates.
In another video that has since gone viral, she is seen tugging at her swimsuit then wincing in pain when it snaps back on her chest.