The 'Smile' hitmaker was looking to buy some presents at luxury department store Harrods when she felt dizzy and fainted, reported Daily Mail.
But she knew what was needed to give herself an energy boost - a cheese and ham toastie.
Allen also let her fans know what some of her relatives will be opening on Christmas Day.
"On the bright side, managed to pick up Professor Snape Wand in Ollivanders box and Slitherin pen for Nephews Xmas present. #luckyboy (sic)," she tweeted.
Allen is pregnant with her second child with husband Sam Cooper. She is believed to be due next month.
She announced that she was pregnant again in July, tweeting, "Thank you for all your congratulations today. I honestly couldn't be happier."
It's a big day for Allen and Cooper on Sunday - they celebrate their daughter Ethel's first birthday.
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