I concur with what others have said about pecking order. I have had that happen with a hen where they all got along fine but after a while I noticed she was always the last one into the coop and it got later and later every night. Soon she would just hunker down outside the coop in the dark and I'd put her in each night so she wouldn't become a raccoon or coyote snack. Then in the a.m. she would race out of the coop squawking like she was on fire. Turns out she was just trying to get away from the mean ones. Hope you get your figured out!
A huge thanks for the many kind comments and welcomes. I haven't had any further problems yet. They all look so alike that I couldn't even tell which one it was when I let them out the next morning.
@4-H Chicken Mom, Yes I do have roosts in the coop/stall but they don't sleep on them. I do see them on there when I go up in the morning to let them out, but usually there's only a couple on them.
If you are concerned you can separate her from the others for awhile to observe her laying , feed and water consumption, and movements. however if it is just that she on the bottom of the pecking order that will be worse when she is reintroduced. Sometimes it is best to keep a close watch and see if she is bullied by the other chicks.