I'm underweight (5'3 & 93 lbs). Now I had a stomach flu past 2 days and my weight is 90 lbs, I feel weak, what is the best to eat during recovery?

Food. Continue with a varied diet that contains all elements of foodstuffs. Vegetables, fruits, meats, and concentrated carbohydrates (potatoes, etc.) are the way to go. You may just have to eat more. Good luck and best wishes.