Nope, an infertile egg would never do that. People incubate those for three weeks all the time. Most of us candle at 7 days and remove them. I kiddingly call them lava lights, as when you candle, there's nothing there but a yellow glow. This OP has no rooster.
I don't know. Just trying to make sense of this, don't mind me. 
In the lack of a rooster, often when a hen has an ovary issue, a hen might actually start acting like a rooster and start mounting other hens. This can happen in reverse as well but this is also generally rare cases. That could? be the reason you look to have a fertile egg. I've heard from some farms that out of curiosity they discovered refridgerated eggs can still be incubated and hatched in some cases. Either way though, I think others are right with the advice to look for infection or health issues and watch her laying and eating patterns. It is more likely she has an issue than got fertilized by another bird in this case, especially if she is not free range.
Sorry, I was a little confused by that post.
Look under "blood spots". Sometimes those can be massive.
That odor concerns me. Was it a rotten egg smell, really strong? That means bacteria got inside and had a few days to multiply. Could the egg have been hiding in the nest bedding so it was missed for a few days? If it was not a rotten egg smell I'll go back to the blood spot theory. Or maybe meat spot.
Ok thank you! So far it’s just the one egg.
Yes I started to wonder if it was in the coop for a time buried. My husband supposedly checks in there and I do when I clean. No telling. My husband put nesting boxes in the corner of our kitchen door porch. He’s got them laying there. Delivery right to our door
but that doesn’ mean they don’t find elsewhere! 
