Zombies having more strength and not being able to die

So I was mining and I came across a zombie. I had full iron armor and full health at the time, and I took out my sword and starting hitting it. I was still full health when it got a shot off on me and it killed me instantly. I respawned and went back to the spot where I died, and I grabbed my stuff and put my armor back on. I got back out my sword and got more shots on it. I probably hit it enough to kill it 3 times but it never died. It then hit me and killed me instantly once more. Still full iron, still full health.