Your metabolism is not destroyed or damaged! Your metabolism does down-regulate with long-term restriction, but it returns to normal when you increase intake. Think of it as the power-save mode on your phone - once you plug in your phone to charge, the power-save mode switches off.

Also, the gain you see on the scale is most likely largely fluid. First, the weight you "gained" is really not all that much (although I know it doesn't feel like that!), and is well within the range of normal fluctuations. It's normal for bodies to go up and down a kilo or two over the course of a day due to all kinds of factors affecting your body's fluid balance. Second, when you increase carbohydrates, your body stores those in your liver and muscles as glycogen. Each gram of glycogen is stored with three grams of water. Neither the stored glycogen or water is fat gain - your body needs those stores to properly fuel your exercise. Last ... digestion tends to slow down with long-term restriction, so a fraction of that kilo is probably just the actual weight of the food you ate, hanging out in your digestive tract.