Alright I made a huge wall useing the new block thanks to creative mode...

You were attacked by Silverfish. Silverfish are a new mob (new to the 360 version) that can hide in Stone, Cobblestone, and Stone Brick. They can also enter all variations of those blocks, but the block's texture will turn to one of those (if a silverfish enters Mossy Stone Brick, then the block will appear to turn into regular stone brick). They only naturally spawn in Strongholds (in the newest PC version they can also spawn underground, albeit very rarely). When you mine one of their blocks, they will pop out and attack. If you attack one of them then all nearby Silverfish will break out and attack you as well.

I don't know if Monster Eggs (the Stone/Cobblestone/Stone Brick blocks that contain a Silverfish) were added yet, as they were not added to the PC version's creative mode until much later. This means that you were either in a Stronghold or near a Stronghold.