If you can look into her throat, and you see some tiny red lines, that look like tiny strings, then it could be gape worm. Alot of the time I do think they get something stuck in their throats, making it feel funny, so they do that neck stretching that you describe. If you see more chicks doing this, then it could very well be gape worm.
All you can do is keep an eye on her, and this behavior might stop. I know you don't see anything but it might be way down in there. I wouldnt treat for gape worm if she's the only one.....
Just my humble opinion.