Antibiotics should not be used unless you know more about what might be the problem. Do your remaining chickens have any symptoms? Amoxicillin is not the right choice for fowl cholera or respiratory diseases or awelling due to respiratory diseases. Without pictures it is just a wild guess. Fowl chlolera is usally treated with sulfa drugs, such as bactrim, sulfadimethoxine, or similar. Respiratory diseases if caused by MG, can be treated with tylosin and a few others. Coryza is treated with a combination sulfa and tylosin. If you lose another, please keep the body cold and contact your state vet for a necropsy. Sorry for your loss. In NC, you have a choice of where to send a body for necropsy, and the list here gives contact info:
https://www.metzerfarms.com/PoultryLabs.cfm