Most of that weight would be water. We lose a lot of water during our 8 hours of sleep, which is why some people need a glass of water by their bed and most of us feel dehydrated when we wake up. 2kg is a lot, but given it's usually about a kilo that you lose overnight in water that's not too far outside of what's normal. If it's something that concerns you you might want to talk to your GP about it.
Also, how much food did you eat during the day/especially at dinner? It takes a few hours for food to leave the stomach and make its way down the intestinal tract.. much of that food gets turned into energy, used for keeping you alive. Some of it ends up travelling the whole way, and becomes poo... if you factor in the water losses, losses from food being turned into energy, and then actual losses through going to the toilet etc, 2kg can easily happen - especially once you factor in the variability in most sets of scales (even good scales have about 10% inaccuracy range!)