Fanning, a member of the jury, collapsed out of her chair during the Trophée Chopard dinner, according to outlets.
“Oops, had a fainting spell tonight in my 1950’s Prada prom dress but it’s all good!!” she wrote on Instagram with a photo of her giving a thumbs-up. She indicated in her hashtags that her dress was too tight and she was dealing with her “time of the month.”
According to Variety, the 21-year-old “Mary Shelley” actress was “helped to her feet” by her sister Dakota. Actor Colin Firth, sitting nearby, also rushed to the table to offer assistance.
Security then escorted the sisters out of the room.
Here’s Fanning at the event earlier.