Getting hit by invisible / non-existent mob

I keep getting hit by something. The game sort of treats it like a mob. It moves, attacks, and I can't place blocks on it. However, its completely invisible and I can't hit it. The only things I can do are trapping it and blocking with my shield. I was using a texture pack, but I doubt that the texture pack is causing it since I've used that texture pack for quite a while and nothing like this has ever happened.