Dog puking up white mucus stuff?

My West Highland White Terrier puppy and I were walking down the col-de-sac. Very abruptly, he stopped walking and tugged against the leash. I stopped to see if he was going to the bathroom. He started crawling around funny, then started making gurgling noises. He threw up. I decided to bring him back to the house and he threw up on the way back. In the house, he threw up twice again. I cleaned the puke and gave him a tiny bit of water. He seems fine now, but what was the cause of this?