Something similar happened when Hamish was a pup. I had taken him for a walk and he started gagging. I ended up having to pull it out of his throat as it was stuck. I couldn't really see it because it was dark but it seemed to be grass and bits of wood maybe. It smelled horrible. he was fine afterwards. I assumed it was just a collection of stuff he couldnt digest. He was really into eating a bunch of junk at that stage.