well, the main possibilities are.........
1) she's starting a moult. perfectly normal, nowt to be done, feathers all over the gaff

she won't lay any eggs while moulting. most chooks moult in the autumn but my Boudi has three or so partial moults at varying times just to be awkward!
2) she's getting broody. perfectly normal although inconvenient. broody hens will pluck some feathers in order to have their hot little belly closer to the eggs

if she's staying on her nest box, grumpy when you try to move her and lies kind of flat...like a big feathery pancake she may be brooding. remove eggs promptly each day and let me know and I'll pm you with the whole broody hen lesson
3) one of the others is plucking her

not so likely cos this would normally show first round her bum but it's still a possibility. watch out for any sign of bullying.
4) red mites or other beasties. check beneath her wings and around her vent for lice (biege to orange, look like tiny grains of rice) and check in the nooks and crannies of her coop for redmites (little black dots that splat red when squished with your thumb nail

) either of these beastie types would be unlikely at this (cold) time of year and would affect all your hens not just one but it's always worth a check cos they're 'orrible.