Getting Random Damage From Invizible Zombie on Nether (SkyBlock)

Hello ! I was playing on Minecraft 1.6.2 on SkyBlock 2.1. I went to nether , on skyblock nether world is empty , so I built a road with cobblestone half slabs to prevent pigmen spawning. when i walk on that road or when im on nether i get random hits by nothing I also made some screenshots. when i'm getting hitted i always change my gamemode to avoid being killed, but today i decided to let that "Invizible" Mob to kill me and when i got killed on chat it said : eltonalbania was slain by Zombie. its so weird , i know that on nether isn't any zombie i never spawned them with eggs, this happens to me every time i go on nether , also this happened on my 2nd pc too , See The Screenshots Bellow, Thanks