mystery green egg - where did it come from???

I have four chickens - two Buff Orpingtons who lay, and two others that hatched in June, so not laying yet. Doreen has been broody for a few weeks, but I think possibly still laying. We get at least one egg a day and I think sometimes two (it's not just that I forget to collect). At the end of July, I got busted by the bylaw officer and had to get rid of my two EEs and two RIRs. Back in May and June, Gloria was broody. Since I have no rooster, I got her some hatching eggs. She screwed it up, though, and the nest was abandoned. Creeped out by the idea of possibly seeing partially developed chicks, I avoided tossing the eggs and eventually forgot about it. Gloria's eggs were outside the main coop. Furthermore, they were all brown. To compensate for her eggs not hatching, I got 6 chicks (4 of which were also given up when I got busted by bylaw).

This was all relevant background. Today I collected eggs. There were two - one brown, one green. What the...

I'm making a meatloaf. I cautiously cracked open the green egg and my thumb immediately got thick yellow ooze onto it, like a broken yolk. Except I don't break yolks, and I didn't crack it very hard. I opened this egg into the sink, and the entire thing appeared to be yolk inside. I opened it up completely and a small (maybe 3/4" round) dark blob fell out, but slipped down the garbage disposal quickly before I could really see it.

WHAT WAS IT??? Was that a rotten egg Doreen found in the coop? If so, why didn't I smell anything. Was it a so-rotten-it-changed-color fertilized egg that she somehow moved into the coop? Can chickens even do that?

The brown egg was normal.

 

Except we've never had a rooster...?