Why does a perfectly normal mirror in my bathroom make me freeze for a second every night

Okay so this is silly but every time I walk into the bathroom at night and catch my reflection in that big mirror, I get this tiny jolt like I wasn't expecting to see anyone there. The light is on, nothing spooky is happening, and it's obviously just me. Still, that half-second of 'who is that' hits before my brain catches up. I end up laughing at myself right after, but the initial reaction keeps happening. I've tried turning the light on before opening the door, looking away, or just telling myself it's ridiculous, but the reflex is still there. Does anyone else have some random everyday thing that gives them an automatic reaction even when they know it's completely harmless?