Do you have any antibiotic prescription medicine lying about? It can be from when you or someone in your family was sick and it's left over from that. If I were to take a wild guess, an infection is what I would come up with. Sometimes all we can do is guess, and hope we get it right.
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Please state your location on the planet. It has bearing on possible causes of disease and treatments available.
It's not possible to tell you what is wrong with your young chicken. But we may be able to help you rule things out.
Crop disorders can make a chicken very sick and even kill them...