I have always been told that chickens should not have access to long, lush grass, hence I have goats and tame cows eating down the individual breed pens, and the grass where my free-rangers go before the chickens can eat it. Also, have your chickens been wormed recently - I tend to worm mine Spring and Autumn, and this seems to keep them in reasonably good health. Don't worry about the squitty bum, though, as chickens do seem to get that occasionally!
Catherine.