Stimulus changes our heart rate and other physiological process. We then perceive these responses as emotions.
Emotion then is the 'perception' of our current physiological state.
Sometimes we don't even have time to perceive because our reactions are too quick. In times when the stimuli are present for a while, we realize we are scared or angry but this is because our body has already reacted. You aren't aware of these physiological reactions, necessarily. You are just aware of this emotion. You don't have to interpret your body.