Terraria – I can’t find the magic mirror

Chances are they are spawning above you in the mine. Any kind of wall (stone or wood) except dirt will stop monsters spawning. I believe the walls are what stop monsters spawning unlike Minecraft's light level system.

If you've just dug straight into the earth, use the hammer to remove the dirt walls and replace with stone.

The other option is there is a blood moon on and the zombies broke through your house.

From Terraria Gamepedia:

A Blood Moon will only occur if at least one player in the world has over 120 life, and only on nights when a moon is present (any night except a new moon). Given those criteria, a Blood Moon has a 1 in 9 chance (11.11%) of occurring.

Following the criteria above, there is a 1/9 chance (or 11.11%) chance of a Blood Moon occurring.