Loaded up my single player survival world.  Walked straight over to my nether portal.  When nether loaded, I was immediately killed by a swarm of Phantoms.  For the next ten minutes I went back in to attempt to recover my items.  I was able to run around a tiny bit in between deaths and there were literally hundreds of them, looked like bats hanging from the ceiling, they would immediately swoop down and kill me.  I'm not exaggerating.  If I had to quantify the number of phantoms that were in the nether I'd estimate 100+

After about ten minutes of dying and running back, I was finally able to find a sort of cave in the nether.  I couldn't come out because I could hear them but I could see my items through a one block hole lookout.  Once my items despawned, so did all the phantoms.  They all just disappeared. 

Pigmen were not aggravated at all during all of this.  I had slept the night before AND at some point during the ten minutes of me dying and running back.  (I thought sleeping might make them go away so I could recover my items.  I'm sorry, I couldn't get any screenshots.  I'd enter the portal and have maybe 2-3 seconds to run before dying.  I don't know that I have any reproducible steps to list to recreate this as I have NO IDEA how this even happened.  I just needed a little nether quartz and ending up losing all my diamond armor, and tools.  Such a strange and hugely disappointing bug.