Did something die in here?

There is a horrible smell in the entryway of my apartment. It hit me like a brick wall when I returned home from work. I can still catch a nose of it from my apartment 2 floors directly above. What is it? I've never had the occasion to take a whiff of a flaming bag of human scat, but I think I've just come close enough. I have no idea what it is or where it's coming from, I just want it to go away.