Chicken standing still

Please help with our poor bird. Three days ago she was standing still in the garden and didn't run to get her snacks (unheard of). She had a very full crop and I could feel the grain inside it that I had fed her 24 hours previously. I rang the vet (who by her own admission is not an expert), and we decided to make sure that she drank some water and see how she was in 24 hours. Towards the end of the afternoon (after a bit of gentle crop massage and natural yoghurt) she started moving moving around, even though her crop was still full. The next morning she was fine - empty crop and running around feeding and drinking as normal. This was great, but this morning, she is standing still again. This time she has an empty crop and although she has picked at a bit of oats, honey and warm water, she has returned to the garden and has continued to stand still. Her poo this morning is frothy and white, with just a bit of dark matter in it.
We are at a loss now - we had our first chicken death about a month ago, the only symptoms being having spent 2 days standing still, then falling over dead. Both of these chickens are/were just under 3 years old, and are the only birds we have who have never moulted (seems strange, though mey may not be relevant).
Please help - we don't want another death from standing still!

 

Hi there, thanks for the advice - I just put her in a nice cosy box (she had only moved about half a metre since my last post) and was waiting for the scrambled egg to cool a bit before I gave her it, when she 'laid' an egg white, a bit of yolk and a bit of shell. She ate the shell straight away. While I was clearing it up I put her outside with the scrambled egg where the others couldn't get her. She ate some of the scrambled egg, drank a bit of water, then 'laid' the yolk. I was watching her and saw a bit of of the shell poking out of her, but then go back in again. She is now stood 'flaps down' as though she is trying to lay the rest. She has always been a good layer and has had no obvious trauma recently. She is very tame and we handle her a lot, but we are always very careful not to squash the birds. What do I do now? Please help!

 

She must have passed and eaten the rest of the shell while I wasn't looking - she seems fine now in herself, but still has white poo. Does anyone know if this can be just a one-off, or is it a sign of somethiing else?