When you say you don't have the same problem on a real bike, do you mean compared to riding a real bike within a day of riding the stationary bike? Or are you comparing your experience to riding your real bike a few weeks/months ago?
Yawning is generally a sign that your body is trying to pull in more oxygen. Sounds like you are breathing differently when you're inside vs. when you're outside. Are you tensing up when you're on the treadmill or inside bike?
