I attacked Zombie Pigmen in the Nether and died. Upon returning to the Nether (the fortress I was in was right next to the portal, and next to the Zombie Pigmen), they immediately attacked me without me doing anything to them. I kept returning after death only to have them attack me again. Even new ones that JUST spawned in became aggressive and now I can no longer enter the Nether without having a swarm of about 45 Zombie Pigmen after me.