For me it used to be hyperventilation. This later morphed into a feeling that a new forum member best described, “the boiling of your blood.” At a very light level it can be as light as a head change, you just feel different slightly. At the extreme level its hyperventilation. It sorta feels like there is a vacuum sucking on your veins, or almost like the air pressure feeling in an airplane.
Other symptoms that can occur after eating bad foods include gas(bloating), IBS, headaches, tremors/muscle twitches, runny nose, heart palpitations, mucus production, etc.
Remember, not everyone had the same symptoms before the diet, so this reaction symptom can vary; or you might not even have it.
If you feel really great, go ahead and try an apple. You may feel a big sugar rush, it may set you back a little(few hours), but its definitely not the end of the world and your body can handle it. If you were to go onto stage 2; I would slowly add foods that seem safe to you. Save the wheat/grains and dairy for the end.