What Kind of Spider Did I Just Kill!?

I was in my basement bedroom which has wooden walls and a cement type floor. Anyways I was walking out of my room and near my door was a spider about the size of 4 quarters put together in a square. I smashed it with a slipper and when I did a whole bunch of mini baby spiders started scattering and I had to spray them with bleach to kill them all. I never caught the color of it but it was a dark colored spider. I'm 15 and I'm scared to DEATH of spiders. Now that I saw it I'm paranoid worse than ever. What kind of spider do you think it was?