My cat just had a terrible reaction to his annual shots

[Skittles]

after I brought him home he vomited his food which I didn't think anything of - Clancy gobbles his food and has always been prone to barfing - but he kept retching and eventually was tossing up foam; then he started wailing really loud and twitching. I called the vet who said bring him in; the vet said he's going to give him a shot of something - I think he said cortisone - and keep him until five. He didn't sound that alarmed but I am freaking out. Clancy is six and has always had annual shots and nothing has ever happened - any of you know anything about these reactions? :(