As SIENNA MILLER exited Anthropologie in Madhattan, a femme fan made her smile with a reference to her romantic reunion with on-again-off-again fiance JUDE LAW, gushing: “You’re just glowing!…You must be very happy. I hope you have lots of babies.”

But Sienna’s smile faded fasterthanthis, says My SpyWitness, when the snarky sow winked and added: “Just remember, DON’T HIRE A NANNY!… Or make sure she’s an old, fat one!”

Said my source: “Sienna just stood with mouth agape as the woman split. You could almost see bad memories flooding back as color drained from her face.”

Memories like…the “Jude Law: Love Rat” headlines triggered by the naughty nanny of his kids by ex-wife SADIE FROST, who revealed he’d bedded her repeatedly – including once on a pool table!