Wow! Congrats on taking a first step to a healthy life by losing 70 pounds!
That stomach flap in my opinion is composed of remaining fat AND the excess skin your body had to grow to cover the fat that was in this area before you lost it. You are young, keep exercising and eating healthy and this excess skin will eventually grow smaller. It will not happen overnite though.
I would say to start with eliminating dairy from your diet ( milk, cheese, ice cream, etc) to see if your midsection decreases in size. You may have bloating due to dairy. I personally have this issue and have learned to stay away from dairy.
Are there any foods that you eat that you feel a little "ill" from? If so, this may be a food allergy. Food allergies can result in bloated midsections.
Hope this helps. Keep up the healthy lifestyle.