Billie Eilish is celebrating ... included a throwback photo from a night in the pool, with Billie's then blonde locks in an updo as she clicked a selfie wearing a black swimsuit and a gold chain.

The 22-year-old singer took to social media with several photos ... to her bikini snap, while a third added: "Billie Living HER Life! What a time to be alive!" MORE: Billie Eilish makes surprising ...