That was fast! Just four weeks after giving birth, the 5-foot-7 singer flaunted a svelte frame after her hubby’s Carnegie Hall concerts in NYC February 6 and 7. Her secret? “Beyoncé worked out while pregnant, so she didn’t gain much,” says an insider. And though she isn’t strict about her diet, the star was spotted sucking lemon slices at one postshow 40/40 Club bash.