First off, know that you have lost a lot of water weight, intestinal contents and a couple of other more minor temporary losses, all of which will be returning within about 2-3 weeks of refeeding. In your case it could be 15-30 pounds of it. 20 pounds of water returning has a way of filling us back out. I look "skinny" to myself right now, but i know it is deceptive.
I once made the mistake after a 14 day fast of thinking i needed to eat a lot to hurry up and put some weight back on (after easing out appropriately). All i really needed was for the 15 pounds of temporary loss to return, then i was to be just right.
If you still find after the 3 weeks of refeeding that you want to add weight, you might consider some resistance training with a 300-500 calorie per day calorie surplus.