Too slim but belly too big. Advise please

5'8" female runner who was 133 pre-preggo weight with a 30" waist, flat stomach, and visible abs. Your ribs and hip shape/size are going to play into your waist size too. I have a very large rib cage and my lower ribs are definitely in play when measuring my waist in the appropriate place. Nothing you can do about that, so it's not worth being miserable to try and cut fat that isn't there. I would encourage you to start lifting heavy, just because it's fun tho!