Before you go looking for the best weight loss pills or chugging smilling teas, do take in mind that the weight gain didn't happen overnight, so neither should the weight loss. Sure, it may have happened over 12 days (or a month, depends how early you started merrymaking) but it will always be healthier to do it naturally first.
Start with eating more fruits and vegetables, and ridding your body of toxins.
Drink plenty of water.
Engage in more activities.
And start reducing the sweets, the salty, the fatty in your diet.
If you did not gain that much to start with, it shouldn't be a problem losing the gained weight. And until then, you can always dress up in ways that create the illusion of a slim silhouette.