It's probably from getting pecked at because it's tinier than the rest.
Chickens will let you know they're sick by their behavior. If this chick continues to be active, alert, good appetite, instead of sleeping a lot and not running around with the others, it's probably okay.
However, the fact that it's smaller than the rest could indicate genetic abnormalities. You'll need to keep an eye on it. Also, you should be observing the chicks for harmful activity and intervene before one ends up badly injured. If you can identify the aggressive chick that may be hurting another, you can discipline it and change the behavior.
http://www.backyardchickens.com/a/aggressive-baby-chicks-and-how-to-stop-the-behavior