Beyonce revealed the secret to her amazing post-pregnancy body as she played four concerts in a weekend at Atlantic City. The Love On Top singer - who gave birth to daughter Blue Ivy in January - wore a number of stunning custom-made Ralph & Busso creations, including a midriff-baring, fringed two-piece embellished with Swarovski crystals for her first gigs since August 2011, reported Usmagazine.com. \"You all have no idea how hard I worked! I had to lose 60 pounds. They had me on that treadmill. I ate lettuce!\" she told the crowds at Revel Beach\'s Ovation Hall. However Beyonce planned to celebrate her live shows with a treat, adding: \"Now tonight I\'m gonna get chocolate wasted!\" The 30-year-old paid tribute to Whitney Houston by singing an a capella version of I Will Always Love You as an introduction for her hit Halo, and performed a selection of hits from her four albums and a medley of Destiny\'s Child songs. Michelle Obama and daughters Malia, 13, and Sasha, 10, attended the show, and were seen dancing to Crazy In Love and Single Ladies. Beyonce\'s rapper husband Jay-Z was also in the audience, and couldn\'t resist tweeting about his wife\'s triumphant return to the stage. \"I\'m gonna say this and then I\'m gonna end mine. BEYONCE is the best performer in the world. Period,\" he wrote.