Hi Satdown
It's quite normal for chooks to stop laying when things change (it gets wetter, or drier, or windier, or calmer.....
) But seriously, a change of home is enough to put them off their stroke.
They will need light over winter, if they are to continue laying. Mine aren't laying well at the moment as they are moulting - feathers all over the place.
The more worrying thing is the 'standing separately and breathing heavily' thing.
Sometimes when chickens are stressed, a bug that is in them all the time can get going and make them ill. Coccidiosis is one of these - they can also get 'the runs' from this and you'll see mucky bum feathers. To treat it you need Baycox from the vet, and you either squirt it in their beaks, or you put it in the water, if they're in a run and you can control what water they drink.
The other possibility is a respiratory disease, again, can be caused by stress - will need antibiotics.
Have they been wormed? If not, you can either buy pellets made with Flubenvet in them (from Marriages), which you feed for a week or you can buy flubenvet separately and mix it into the food for a week (a pain to do evenly, hence the Marriages pellets).
You can try 'nursing' and see if she improves. Your local feedstore will sell 'Poultry Spice' which is a mineral and tonic which can help, you mix it in their food.
Unfortunately, birds that appear ill though are often quite ill by the time they let on
Good luck, I hope she settles in and starts feeling better