Why does Napoleon think he's dying the morning after he drinks the whiskey in Animal Farm?
The Fifth Commandment originally states that "No animal shall drink alcohol," but is broken by Napoleon in chapter 8. After the windmill is destroyed and Squealer announces that the animals are once again victors, the pigs come across a case of whiskey in the cellar of the farmhouse. The other animals hear the pigs partying all night long and even spot Napoleon race around the farmhouse wearing Mr. Jones's old bowler hat. However, the next morning there is a solemn atmosphere on the farm because the pigs believe that Comrade Napoleon is dying. There are even rumors spread throughout the farm that Snowball had poisoned the whiskey. At eleven o'clock, Napoleon even decrees that the drinking of alcohol is punishable by death. However, Napoleon begins feeling better the same evening and is completely fine the next day. Essentially, Napoleon was suffering from a hangover, which made him feel nauseous, tired, and sensitive to light. By the end of the chapter, the pigs begin researching how to make alcohol and the Fifth Commandment is rewritten, "No animal shall drink alcohol to excess."
Under the fifth of the Seven Commandments of Animalism, no animal shall drink alcohol. As with all the other commandments, however, it's subsequently broken for Napoleon's benefit. He subtly changes the commandment from "No animal shall drink alcohol" to "No animal shall drink alcohol to excess." But even this commandment is subsequently abandoned as the pigs start to get a real taste for hard liquor.
At first, though, it makes them feel bad—really bad. In fact, Napoleon's first drop of the hard stuff makes him feel like he's about to die. Sadly, for the farm's animals, that doesn't happen. Napoleon isn't dying; he just can't take his drink. But Napoleon, being Napoleon, doesn't learn his lesson. He is, appropriately enough, rather pig-headed, so he goes on to get drunk more often, as do all the pigs. In retrospect, we can see that this was their first faltering step on the long path to becoming virtually indistinguishable from humans.
He has a hang-over. Being an animal he's never had the experience of drinking alcohol and experiencing the sensations it provides at the various degrees of consumption. He doesn't understand right away what is going on, but later in the afternoon he's feeling surprisingly better. This is also indicative of a hang-over... the morning is the worst part.
The sickness he felt was probably unlike anything he had felt previously and had nothing to compare it with, thus, he thought he was dying.