First question: are you fully rested? Are you getting 7-8 hours of sleep every night? You could just be fatigued....
Yawning is caused by slowed breathing / lack of oxygen.
As with any exercise, you need to seriously focus on your breathing. It's just as important as proper form and technique. Breathe through both your nose and mouth at the same time, taking concentrated, deep breaths. This will help with endurance and energy, and will prevent your heart rate from coming up too quickly.
If you're still having problems, it could be a heart/ blood pressure issue and you should see your primary care physician.
"I have a personal relationship with food. I love it, but it's on the fence about me." ~Kathy Griffin
"If there was a famine, we'd be the last to survive." ~My mother
| current weight: 137.8