With chicks that young, the two main fears are mareks and cocci. To prevent cocci, always start your chicks on medicated feed, keep them cool/well-ventilated, clean your coop often, and keep the waterer clear of any chicken poop. I've never dealt with mareks, so I don't want to give you any wrong information. Research those two illnesses and see if the any of the symptoms match closely. Another possibility is that's she's just cold. She looks a lot younger than the others, young enough to need a heat lamp or a broody mama to take care of her. Get her under a heat lamp first, then consider illnesses.