I know that chickens wont sit on their eggs until they have a "clutch" but 20 eggs sounds like a clutch to me. Maybe she's not done laying or some of the eggs have died. How long did it take her to lay the 20 eggs?
 
Sorry, but my chickens never got a bald spot so I have no idea.
 
You can ask vets at this site for free:

http://
pet.justanswer.com/veterinary?r
=gapet&gclid=CN67m8ali5
MCFQRjxgod9BP4hQ

 Hens get bald spots from stress and in-fighting. I'm sure you've heard of 'pecking order'. It's quite literal. Perhaps the two have suddenly decided to fight for top chicken, or perhaps there is predator nearby increasing their anxiety. The constant egg laying must be stressful as well. Look into ways to slow her egg laying. I've heard of a similar problem before. When a hen comes to realize none of the eggs are hatching, she lays more and more and fails to properly care for them. It is a nesting problem. Remove the eggs for now and let her grow comfy on an empty nest. See if her egg laying slows any. If it does, let her sit on one egg, continuously, remove the others. It might calm her. This is purely theoretical, based on a similar situation I observed.