Help! Sick Chicken

Hi,

I got 4 new girls on 23/6/12, 2 light sussex & 2 Rhode Island Red at POL.  There seem to have been settling in fine they are currently quarantined form my other 6 girls.  I put them straight on to layers pellets with flubenvet.  This was good idea as there poop looked like it contained round worms,  they are also on poultry booster vitamin supplement to aid them through the onset of lay.

Today however the flightest Sussex was just laid in the coop, there was some very loose droppings with red in (presumably blood).  I put my hand in and she did not try to get away from me.  I picked her up and it seems that her head / neck is the only thing with any strenght in it, her wings are drooping she has no resistance from her legs.  Up to today she has shown no ill affects.  She is alert her eys are bright, she has no nasel discharge,  I have checked her for mites and can see nothing

I am very worried that how quickly she has fallen ill. 

Could this be Coccidiosis?

I would really appreciate any advise from your knowledgable people.#
Thanks