How can I get a flat stomach in 2-3 weeks?

I am 5′ 5" And I weigh 138 lbs… I am going to my brother in law’s wedding in 3 weeks and would like to know how I can get a flat stomach in that short of a time. I used to have trouble gaining weight until I got married, since my wedding I’ve gained 20-25 lbs… Most people say that I needed to gain the weight; but I don’t want to loose my flat stomach and not be in good shape. Any suggestions would be appreciated