Your conscious mind is the thinker.
Your body is the lie detector.
Your subconscious mind watches for clues.
It doesn't matter how hard you try to pretend or 'fake it til you make it' - if your body tenses or flinches while your mind tries to play it cool, your body has called out the lie.
Best policy?
Observe the physical reaction but don't hold onto it or react. The worst thing you can do is add emotion to a negative feeling - in that case, you're adding fuel to the fire.
Stand back, let the feeling pass, unattached and unemotional and allow the subsconscious to imprint your new state.