Going into winter, you may feel the need to up the quantity of hay, but there's no need for grain, as it's fibre, not energy that warms a horse up. I'd be especially careful of grain, especially sweet feed, as she's not getting any exercise. If you're feeding her her grain ration in only 1-3 feeds per day, this will also have an effect.
While the pics don't give away much, the second pic seems to show her front feet may also already be problematic. Happy to give you a hoof eval. If you like to send pics.
You can learn more about how diet effects the feet at hoofrehab.com & safergrass.org among others.