I made cornbread four days ago. It tasted good initially, but had a bitter aftertaste after three days. What would cause that?

Cornmeal goes rancid VERY quickly. Store your cornmeal in the freezer. After baking cornbread, either eat it or freeze it within 1 day.

This happens with every recipe I've ever tried. In recent years, I've been baking cornbread in mini-loaves and freezing the ones we don't need that night. I defrost and toast, and slather on a touch of butter, and voila! Nearly as good as freshly made (actually better, to my mind, because I love the taste of toasted cornbread). ;o)

I've had this happen also. A couple of ideas for the remains - make stuffing or crumble in chilie

Wow! I'm so glad I read this. I thought I did something horribly wrong - after 2 days my delish cornbread turned into bitter yuck. My only addition to the knowledge base is that both times this happened to me I used creamed corn which kept it REALLY moist. Maybe some chemical reaction needs high moisture? I've never had dry/crumbly cornbread go sour. I hope someone can put some science to this...!

Not sure the cause, but yes also find same bitterness (or almost a metallic taste) after first day.
Solution (or coping behavior) is to make just what you can eat fresh, and freeze the rest to use in baked dishes like a pudding, cheese casserole or instead of toast under eggs.

I've had that happen to my cornbread too! I think it probably is some sort of spoilage, but really don't know precisely what it is. I have tasted bitterness caused by too much baking powder in other things, but it's a somewhat different flavor and as you say, is there from the start.
I now usually freeze whatever won't be eaten in a day or two to avoid this.