When you are concerned about your chicken having a health issue, the most important indicator is behavior. If a chicken is behaving normally, there usually isn't cause for concern.
A sick chicken will first of all become lethargic. It appears in the eyes as a dullness, and even some redness will show up in the rims. They will look like the chicken has a bad hangover.
This may be followed by runny, watery poop and lack of appetite.
If this chick isn't eating, perhaps the feed is rancid. Use your nose. It should smell fresh and grainy. Check the chick over for parasites. Look around the vent area for crawling things.
The reason I believe you have a pullet is the pale comb. By six weeks, a cockerel has a noticeably red comb.