need help sick chickens an i already lost 2.

[bassbass7]

I had two 13 to 14 week old chickens that died . The first one I seen no signs that they were sick. I can home from work and it was laying face down not moving . So the I kept a watch on them a day later I noticed one was acting funny breathing heavy an not moving around so I put it in its own pen. Day later dead. So I went an got antibiotics this week none died yet.but I noticed a lump on one off there necks that looks like a tumor . What do I do. That chick is 4 months old. I also have 6 other chicks in the same pen. Please help.