Invisible, Invincible Mob

Summary:

Unknown mob, possibly a Zombie, attacked me while in a cave mining. I had no audio cues and there was no visible mob though I was taking damage and being pushed accordingly. Using a shield would cause a pause in the attacks, hence my belief it was a zombie, like it was pushed down to a hole. Upon attempting to fight back, no damage could be done to the unknown mob and I proceeded to flee.