old timmer told me one time on cold nights a hen will sit on the nest all night idk if its true or not but makes sense.

Hens sit on the nest all night anyways if they have started incubating.. If their clutch wasn't complete and they sat on the eggs during a cold night, that would start the development process of the eggs already laid, and the ones still inside her would be at a different development stage.
In my area they have not started incubating, I would say about April 22 most will start to set.