02/04/2013 AT 12:50 PM EST
Well, that was fast! Just seven months after welcoming son Brooks
, Molly Sims posted a photo
of her svelte body in a black two-piece bathing suit while on vacation in Mexico Saturday with her girlfriends.
"Finally bikini ready & pulling out those Sports Illustrated
swimsuit poses, just in time for my bestie Lauren's big bday!" she Tweeted.
Although sad about leaving her baby boy
at home for the first time, the new mom managed to have a good time celebrating her close pal at the Viceroy Riviera Maya
"Girls weekend, so fun! Love the sun. Love the beach," the model and actress, 39, wrote. "Best girlfriends getaway ever! Viva Mexico ... OLE!"
While not in a rush to shed the weight she gained during her pregnancy, Sims did enlist fitness trainer Tracy Anderson
to help her get back to her pre-baby shape. But that doesn't mean it was an easy process.
"You have to think about a baby first, and you have to get off all of that fat and weight – and that's a whole thing on its own. It's literally the hardest thing I've ever done," she told PEOPLE recently