What Causes Continuous Yawning?

For the past hour i have been yawning constantly,but most of them are half yawns. About an hour before I was fire fighting which was may more action than normal. And I had to push my car on my own. Felt like superwoman at times. But came inside and have this constant yawning feeling and then the yawn is only half. I am 49 yrs female and have a good bill of health from my doctor - 2 weeks ago - only problem he said was i am too anxious. Been doing breathing exercises.