Poorly chook

One of my 40 week old black rocks has gone off her food and off lay (hasn't laid for days - usually 5 times pw). On Monday she didn't want to come out of the coop....for a while she had runny yellowy poo with little pellets of solid - now not much poo at all.
She hasn't eaten much since Monday - a spoonfull of scrambled egg from time to time and some mashed up layers pellets. She drinks when I coax her.
The strange thing is she's making a sort of clicking noise when she pecks, eats or drinks - a bit like castanets. No bulging of the crop and I've massaged her neck a bit which she seems to like - as she does a warm bath and blow dry!
I have her indoors in this cold weather and apart from sitting down all the time she's quite alert - cocking her head and inquisitive. Any ideas? I'm not really inclined to make a trip to the vet, so if this looks terminal I'll do the deed. But would like to nurture her if we're in with a chance.
Thanks.

Thanks Joyfull. I wormed them with flubenvet a little under 3 months ago. If it's time again, should I wait till she's well?

Egg bound? How far up should I feel? Not done that before! Is it obvious to tell? Maybe I should give her another warm bath - she's a bit mucky round the nether regions!
Thanks again, BG