You know the moment the coffee hits your gullet, near instant regret hitting you in the head. Your vision almost seems to blur, heart pounding in your head and breath turning heavy. For all you know, the feeling of your insides shifting is literal, though your body changing must be all in your mind, surely.
There was no way you were watching some sort of muzzle grow out of your face or your ears were lengthening until they drooped. It must be your imagination when the fuzzy feeling over your skin turns literal, and your legs painlessly snap into digitigrade. The coffee must have been bad, somehow.
Though that doesn't explain why you feel like you're vibrating...