Some chickens have a more gradually molt that's less noticeable. Mine have usually done a more gradually molt, without much of a change in behavior. Some chickens have a hard molt, where they loose so many feathers at once that you can see mostly down or even large areas of skin showing. They look much smaller at that time.
Two of my chickens had a really hard molt this year. They were very skittish and stayed away from the other chickens during that time. Their odd look and being so much smaller made them get picked on a little more, although they weren't damaged in any way. They were understandably paranoid. They did get chased away from snacks a little more, by the other chickens, so I spread the food out more and also fed my sweetest girl on the sly. Blood feathers are also more sensitive while they're growing, than a mature feather. A hard molt means a lot of blood feathers growing in at one time and getting bumped or jostled at all is uncomfortable. When I'm petting my parrot and I accidentally hit a blood feather growing in that I didn't see, he lets me know about it!
When you say she's lost weight, does she just look smaller, feel lighter or are you weighing her? Have you seen your hen eating? Make sure she's not having any problem getting to the food and water, in case she's getting picked on a bit more. You could always feed and water her on her own once a day, to make sure she's getting what she needs. I like to feed some sunflower seed to the chickens, when they're molting. It seems to help their feathers grow back in a little faster. Some people feed some type of animal protein during a molt, also.
