Is she a hybrid, Gwenneth? If she has been laying really well for two years, she may be approaching the end of her best egglaying days. Like all hens, hybrids bred for egg production have only a certain number of egg cells in their ovaries and because they lay so well in the first two years or so, they use them up faster than many purebreds with less impressive egglaying records, who may go on longer. Commercial egg farms cull their hens at two years old, because they become uneconomic then, although many do go on laying for quite a while and if lucky get rehomed as ex-batts.
As Chuck says, she may well be approaching a moult, and the rest from laying will help her to restock her calcium levels, though as they get older, absorbing calcium seems to get harder for them, it's a complex balance. I've tried various supplements for poor shells in older hybrids but basically none of them worked for long. You just have to accept that, sadly, on average hybrids don't go on as long as purebred hens, though you get all those lovely eggs and no trouble with broodiness whilst they're in their prime.