You could have a bully that is picking on her. I have a RIR I believe she is that is a real witch and loves to pick on anyone that lets her get away with it.
They will peck, they will also pull feathers out too. Sometimes it's a pecking order kerfluffle, or just she's the low man and they pick on her, she walks by and they pull out some feathers. If it's happening, it can kind of invite MORE bullying until she stands up for herself.
Besides separating them, I don't think there is much you can do to alter that behavior.
Molting, each chicken is different. I had one that stopped laying for about a month while she slowly lost feathers and grew new ones. I had one that was down maybe a week and that was it. I had one that I honestly one day thought a hawk got my flock, I walked out and there was this huge pile of feathers there, I was like Oh god no, did a quick count and seen one pretty bald, looked REAL GOOD at her, nope, she's fine, just they dropped out almost all at once! They also molt when they are ready too, there is no set time or season that ive seen.
Aaron