Is my kitten okay if he is vomiting after eating?

My tabby kitten of about 3 1/2 months threw up twice today. This is the first time he's thrown up since we've had him. We haven't changed anything in his diet. He is still active and he still has his appetite. We've been feeding him Purina dry food. And his vomit appears to be mostly orange mush with chunks of dry food in there (I think it basically looks like digested cat food). The only thing out of the ordinary is that I let him go out into the backyard today to play with our adult indoor/outdoor cat for about an hour. You should know that he is a very energetic and equally curious cat. After the hour I called him inside for him to eat and he did as per usual. Not long after I heard him meowing loudly. Then he started retching and vomited. He seemed fine afterward and went to his food bowl to eat some more. An hour later he had vomited again on the carpet. He hasnt eaten since then so as of now there has been no additional vomiting. Is my kitten okay? Do i have to take him to the vet? Can nibbling on some plants outside cause vomiting? I don't want the poor guy to be sick.