There have been numerous reports of molting in less-than-one-year-olds. Climate change or something in the environment, who knows? But it does sound like your pullet is just experiencing a simple molt, partial or full.
You can feed extra protein to help her nutrition. I feed mackerel to my girls a few times a week throughout their molt, and they really love it. Even cat food will help.
If you're going to supplement light, I strongly urge you to tack the extra hours onto the early morning instead of evening. It's much less stressful for the chickens. They go to roost at their regular time, sleep all night through until the light comes on two hours before dawn, serving to merely lengthen their day as it would in summer. It won't hurt to toss some grain into the bedding after they go to roost so they have something to do during those early hours after the light wakes them up.