I've lost about 20 lbs and i'm now at a healthy bmi. My arms are so skinny and even my face is skinnier. I've been a size 0 and still have had the same problem its just my lower abs.. ive been doing cardio and floor exercise, sweating a lot at least an hour or more a day. I was eating 1200- 1500 cals of lean meat whole grains veggies fruits and fatfree dairy as well as small amounts of healthy fat from nuts, avocado, and splashes of olive oil sometimes. I have black coffee or green tea with little to no sugar, no desserts, Then I noticed that after a while I began lying to myself about my intake.. and I was rounding up the 800 calories I took in per day. I've been trying for years on and off and now 3 months to get rid of this lower stomach pouch. It sticks out at the bottom and has a shadow underneath. I cant even up my calorie intake. I just want it gone. Any advice?!
I'm guessing that your body broke down your muscle tissue to replenish it's energy reserves after a workout. This happens when you don't have a good post workout meal. Your body had nothing to feed on so it started using your muscles fibres and I guess that is why you became thin. Considering your lower stomach you essentially want to reduce fat and grow your lower abs right? I suggest you start strength training and not emphasise on cardio so much. And take a protein rich snack/meal post workout. This teaches your body to use the proteins to build muscle without gaining weight. And it's much harder for skinny people to burn calories than people who are fat. That is why you hit a plateau when trying to lose weight. Those last few pounds are always the hardest. Which I think is the same in your case. This is where extra muscle helps. The more muscle you have the more calories you burn. And those last few annoying pounds become a bit easier to tackle.
So put some muscle may on those bones. Hope I helped :)