Hi, i just lost 80 pounds, but i'm having a little trouble here?

for as long as i was born i used to always be fat up until last year when i finally got the courage to motivate myself to get thin. Well i went from 278 to 198 in 8 months which i felt was very quick by the way but i was happy. Now i look great buttt i still have something left around my lower stomache. I don't know if it's stretched out skin or if it's still some fat left. Everytime i go to do a pushup i can see in the mirror that it just sags down towards the floor, not my whole stomache just the middle part where i have stretchmarks. How can i tell if it's fat or stretched skin?? and if it is stretched skin is there anyway i can get rid of it completely and maybe get a six pack over it if i try hard enough? and how long would it take? I'm 18 years old and a guy, so i'm happy i lost the weight when im young and have tons of energy. Thank you all so much :D