Attacked by mobs after entering nether while not being able to interact with game because it said "building terrain."

This has only happened to me once. I entered the nether and heard the sound of a skeleton shooting me, but it would not let me interact with the game. Then, when I could, I had to kill it quickly as it was right up to me after entering the portal. I believe this is unintentional as I've tried to recreate it, and every time afterwards the mobs simply didn't attack me before entering.