Beyonce on the beach in Saint Barths on April 9, 2012. Credit: Landov

It's hard to believe that Beyonce Knowles gave birth just four months ago.

The singer, 30, confidently showed off her svelte post-baby body in a sexy black swimsuit in St. Bart's April 9 -- and with good reason. Knowles hit the gym five days a week with trainer Marco Borges. "We do cardio and plyometric moves like lunges and jumps on and off a bench, the pro tells Us Weekly.

The 5-foot-9 star, who is breast-feeding, avoids processed fare, sticking to whole foods like vegetables and grilled meats. "You don't have to eat only celery sticks," Borges advises. "Be sensible and exercise constantly.

Currently rehearsing for her first post-pregnancy show on May 25 in Atlantic City, Knowles -- mom to daughter Blue Ivy with husband Jay-Z, 42 -- "is loving her body!" a pal tells Us.