Your chick may be too hot. Does she have access to water all day? Does she have a shade cover? How hot does it typically get in the daytime where you are? I'm sorry for all the questions, I am stumped here. It doesn't sound like Marek's but I can't be sure...
Maybe she is just tired? It doesn't sound too extreme. You can try adding some electrolytes to her water. How cold does it get at night? Is she lying on her side when she sleeps, or normally? How old is she?