hey, i am the exact same way. i have never been at that weight were i have had a flat stomach, mostly because i am a short lady and am a bit curvier.. I am 125, and the only difference between my weight now and when i was 157 is really in my face. :/
125 is really small! I'm 160 now, but the problem is I don't lose or gain weight easily. I was 160 during the summer, then during the fall I swam for 1.5-2 hours per day, played soccer for maybe 6 hours per week, and ate semi-healthy (or at least healthier than I had during the summer). This lasted for about 4 months, and I lost no weight. I used to have a flat stomach from maybe when I was 125 (that was a long time ago at like the end of 7th grade, but I hardly had any curves then) until I was 145 or so. The only thing that made me gain those 20 pounds was puberty and just getting more fat on my thighs. Then last year when all these things bombarded me at once and I had sleeping problems and everything, I become 160 and now my metabolism's the same as it used to be and I'm staying the same. I don't think it's super obvious that I gained a lot of weight though because it's all on my butt, thighs, and lower stomach, so now my curves are just more defined.