The 24-year-old British singer performed at the wedding reception of Prince William and Kate Middleton on April 29, and said that it was a hard secret to keep.
'I met William at a festival about a year ago, and they asked me to perform months prior to the wedding,' she told Star magazine.
'So I knew about it for a while, but couldn't tell anybody.
'It was an amazing honour to be asked by Kate and William. I never thought I'd feel that honoured. The atmosphere was incredible and it's something I will never forget.
'I was nervous and overwhelmed. But it was just a fun night, a really great atmosphere. Afterwards, I didn't believe that I'd actually done it.'