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.