zombie pigman

so I made a survival world in minecraft so while playing I went to the nether so I attacked the zombie pigmen then they killled me. But then after a while I went back into the nether for quartz mining then as soon as I entered I got attacked by zombie pigmen. I then respawned and one had come through the portal. Just as I went to pick the items the pigmen which came out of the portal
attacked me.
If this is a bug then please fix this.