I Spy My Enemy at Work

Okay, so I work in a small retail store and there's this one person who always seems to be on my case. They give me side-eye every time I breathe wrong or sigh too loudly. Today was the worst; they literally waited until my lunch break ended just to point out something on the shelf that could wait another day to be fixed. Ugh, it's like dealing with a bully at school all over again. Do others have someone like this? Am I overreacting?