it happened to me before and my neck starts to tingle and then i realize im being followed by a cat (its my cat)
What a strange food allergy you have... But seriously perception is sharpened if you have felt danger in your life (real or imagined). Psyren tells it well.
When I'm sleeping till I check all doors and windows I keep feeling someone in my apartment, my reaction(steal what you want don't wake me) but it always does