[VON KIRCHER]

Every morning when I get up, Zeus, 18.5 week male, will get out of his crate, do a big stretch and yawn. This is normally when he "talks" to me. This morning, however, he yelped while yawning. I noticed, but didn't think much of it. But just seconds later when he yawned again, there was another yelp.
The yelp sounded like if a little paw gets stepped on. His behavior is totally normal, aside from this.
Could this be teething, and when his mouth stretched it hurt his gums? Or could it be that the past couple of days we have been playing more "tug of war"? Any thoughts would be great.