Immediate help for one of my Flocks Needed.

My hens have been vaccinated  for coccidia. They are  7 months old.  They all were healthy up until about 2 months ago.    I noticed one with blood by its vent.  I took her to the vet and he prescribed baytril at a dose of 0.5 ml once per week claiming that it is a very powerful drug that should kill anything. He did not know what specifically was wrong.  He also took a stool sample and nothing came back abnormal.  A month later  "her vent area exploded" .  It was horrible.  Today, another chicken "exploded".  Something is terribly wrong and I need help.  Has anyone experienced this and can you provide me with direction?

I have another coop with older chickens and they are fine.I have never experience this problem before and I don't know how they got this disease or how to prevent it. They eat the same feed and I provide the same water source for them.  The coops are about 50' apart.

Thank You,

NWC

Thank you for your information, but I think it is a different problem.  It's not simply red around the vent area, it is very bloody. It is actually as though her insides explode. The first one affected in this coop was 7 months.  The hen that was affected today is nearly two years old and was laying fine. I moved her to the same flock about 3 months ago because she was  being picked on.  She was healthy and the others in the new coop at the time were too. They are eating and drinking and are alert.  Do you have any more thoughts or suggestions?

NWC