Lifeless hen

Hi Everyone, just wondering if someone could possibly help at this time of night.
Have five gorgeous girls, all perky and lively and laying well - one egg a day from each.  They are all approx. 7 months of age and have laid since we got them at 22/23 weeks.
One of my Bluebelle's however has literally within the last few hours "gone lifeless" on me, runs to greet me and eats when I give them treats, but then just retreats either to the run or a corner of the lawn and just sits there.
If all five have laid an egg, is she egg-bound or can they hurt themselves if she laid a really large egg this morning [one of the Blues has definitely laid a double-yoker this morning].
No answer from the Vet and my girlie hen friends are on holiday - can anyone perhaps suggest what might be wrong and what we do with her?
She lets me pick her up, and as I said she is eatin like normal..........she is one for having a sleep during the day but the others to also, whereas she is just laying on her own.
Am obviously worried as I dont want to lose her.
Any help gratefully received.    She justs looks listless whislt the others are pecking and groping around as normal.
Many thanks

Have you checked her crop? If it feels hard it could be impacted or too soft and squishy like a water balloon could mean sour crop. Also check her abdomen for any swelling and check her vent all looks ok.

I second what daisy said about the red mite, this can leave them feeling very run down.

I hope she's ok, keep us posted x

And lice... don't forget to check for lice! (little whitish/grey things usually under wings or around vent).

Thank you everyone for your concern and support; this morning she was bright and breezy and seemed like back to her normal self.    Unfortuantely I have had to leave them today to attend appointments away from home but the people who look after them with me havent reported anything of concern to me.    One thing I did wonder and was mentioned to me by the lady I bought the girls from, was that they dont like the wind - and yesterday was a shocking day, lots of rain and strong wind.
We have an Eglu Cube, so dont know whether mites live in those or not, but will certainly check.
Many thanks again everyone, really appreciate your help.