Shrunken comb - deep red

Hi,
I'm hoping someone can advise me - I have three hens, raised from day-olds which my son got from his agriculture class at school. They are now about 5 months old and laying beautifully. They appear very happy and quite greedy but I noticed over the past few weeks and especially last couple of days the one hen doesn't feed like the others. At first I just assumed that she didn't like the food choice of grain mix and layer pellets I offered but she also doesn't like any bread crumbs I throw out anymore either. Lately she has also been resting apart from the other two and not as active,going into the hen house early in the evening. Her comb is shrinking and going a deep red colour. She also doesn't seem to be drinking and when I feel her crop, it's soft and puffy with a few lumpy grains.
I feel so sorry for her and have tried everything to get her to eat..
Hope someone can help

 

Thank you for so kindly replying to my question. Unfortunately the poor hen died after being sick for over a week. She started hiding in the bushes but was eating a little though her crop ws just like a soft puffy ball like a marshmallow. I tried all your remedies but still had a sad end. I love your site, so much helpful information. we're in Australia so the climate is very hot in summer and mild winters. it Could have been anything - even a spider bite because we have many poisonous species here.