Help poorly chicken

Can someone tell me what is going on please. A couple of days ago I found one of our four Black Rocks dead. There were signs of blood and stringy bits near its vent so I assumed some sort of prolapse. Now another of our chickens is unwell it has soiling on its vent feathers(white) is hunched and clearly unwell. We have had no eggs today and I cleaned some watery, white /grey poo from the coop earlier. The Other two appear happy and healthly at the moment. Help! Just wanted to add our hens spend most of the day wandering around the garden we have had them since April and have had no problems prior to this. JUST LOOKED AGAIN, SHE HAS HARDLY MOVED THE LAST HOUR, TAIL DOWN, HEAD PULLED IN, BLINKING OFTEN, WHITE DIARRHOEA BEHIND......NOT LOOKING GOOD....IDEAS PLEASE!!!!!!!!!!!!

What Joyfull says!

Good luck with your black rocks and keep us posted

Thankyou all for your ideas. Today our poorly hen still had her tail down, head pulled in, soiled vent feathers but seemed to be moving around a little more. We brought her in and sat her in a sink of warm water for a while, towel dried then blow dried (which she seemed to really like) before returning her to the garden. Her tail was slightly more upward and she seemed a little happier. We bought some wormer and have mixed it with their food and now we are just waiting to see how she is tomorrow. Once again thankyou for your input and I will keep you informed of her progress.

We did indeed buy Flubenvet from our local Countrywide. Spkoke to the lady in the shop who also keeps chickens and had already read about wormers.