Yawning not always equals being tired - might also happen if there is a bit less oxygen (e.g. If your T doesn't open the window between every client) or like it is in my case - due to the stress that is my only reaction if I am really, really nervous - actually then people think that I am very relaxed

But if it happens to your T because she's tired I wouldn't like it neither as I would be afraid that maybe I'm too boring...