why do people make fools of themselves by going on about self hypnosis . . I guy did this in one of those live contest game programmes on tv and he was like "I’m not sure. . I think I’m hypnotized"
References :

Yawning is your body’s way of saying "I need more oxygen". It could mean that you were so relaxed during your hypnosis session that your breathing was shallow and so by the end of the sessions your body is letting you know it wants more oxygen. You can also develop a conditioned response. ie. you may have made it a habit to yawn as you’re nearing the end of your self-hypnosis session. I know I ended up developing a habit of feeling drowsy whenever faced with the certain tasks I didn’t like. Miraculously enough, my drowsiness would disappear as soon as I moved onto something else.

Whether you yawn or not has no bearing on whether or not you were hypnotized. If anything, I would think that it shows you were more likely to have been hypnotized.
References :
http://www.self-hypnosis-downloads.org