Lethargic chook

One of my flock has been out of sorts for a little while. She stopped laying for a while when she went through moult then started up again with the tiniest little eggs very much like her first egg ( I still have all their first eggs, blown out on a stand). She did this for a week or so then stopped again.

She's been fine in herself up until a day or two ago, she's a little over 3 years old now, and this is the first time that she's gone off laying or had any illness for that matter. I did wonder whether that might be it for her laying, I wasn't sure as she's a hybrid white star bird.

Then the last couple of days she's been very quiet and her partner in crime has been very vocal. She's barely left the enclosure over the last day or two and I've not seen her eat at all, she scratches a bit, but she's very lethargic.

Tonight I took her out and gave her a check over, her underside doesn't have a lot of feathers and what feathers she has around her vent was very matted. I gave her a warm bath to remove the matting and checked inside her vent for broken shell. Her vent is flexing quite a bit but there's nothing coming out.

She's drying herself off at the moment, I've made up a little scrambled egg and given her a small bowl of yoghurt, but she's really not bothered with any of it at all.

Does anyone have any advise as to what could be the problem, I thought she might have had a broken shell but there's no sign of that. The matting around her rear would suggest she's had diarrhea at some point, she's drinking and her eyes are bright and shiny but her comb is a little flatter than usual.

My books are useless at under the weather hens, diseases not a problem but poorly chooks, they are completely rubbish.

To give you some background to her, she's one of 2 hens in an enclosure of about 35sqm. They are on bark mainly but have access to the garden periodically when I'm home. They have layers pellets, dandelions, a little corn as a treat from time to time. Access to oyster shell and fresh water.

I'm pretty religious about cleaning, so there's no sign of bugs in the coop and I replace the bark every 3 months or so.

Any initial thoughts or questions would be appreciated as I'm a bit lost as to what to do really.