odd eggs from one chook

At least, i'm assuming it's the one chook! I have three hybrids, they've laid all winter and usually all three eggs are brown, with one speckled. But for the past 10 days, one egg has been coming out white, with chalky looking ends. The hen laying this skips days every now and then, and on two occaisions has then produced BIG double yolkers. This morning, there was a white very thin shelled egg that was perfectly round (normal inside though). They have oyster shell to pick at all day and are fed Organic layers pellets, and free range every afternoon, so I don't think they are missing anything diet wise. The onyl thing I can think of that may be disrupting her laying is that I think these eggs come from the smallest hen, the one who is always last to lay in the pecking order, she often gets to lay in the coop rather than the box if they other tow are occupying it. Any ideas? They all seem perfectly well otherwise, healthy colour and crops etc. They haven't had a moult yet though, they were POL in September.

Hi Sheridan, do you give them grit has well has oyster shell and do you give them any veg, when was the last time you wormed them? Pol

the diet seem fine, one off mine would lay a soft egg the day after it had rained

I know this wont be of any use but maybe its just the way shes made?  She might just work that way with her laying and as you have said none appear ill and all are eating a good diet.