The `Smile` singer - who is currently pregnant with her second child with husband Sam Cooper - was Christmas shopping at London department store Harrods when she came over all dizzy and fell to the ground.
Fortunately, Lily was fine and decided to tuck into a croque monsieur - a toasted cheese and ham sandwich - to give her some much-needed energy.
She tweeted: "Just fainted in Harrods. #croquemonsieur needed."
However, Lily - whose daughter, Ethel, celebrates her 1st birthday on Sunday - did manage to get some shopping done and picked up some gifts for her nephew to open on Christmas Day (25.12.12).
She wrote: "On the bright side, managed to pick up Professor Snape Wand in Ollivanders box and Slitherin pen for Nephews Xmas present. #luckyboy (sic)"
It`s not been a good few days for Lily as she recently bit the inside of her cheek during a recent meal out and was in a to of pain for hours afterwards.
She tweeted: Went for dinner with pals @sebchew and @msaimeephillips and bit off the inside of my cheek. #fuckingpainful (sic)"