I was going to say that it's the stall bedding. Which makes me ask, why is it bedded? If your horse can come and go freely, won't they lay down outside when they want to lay down and sleep? Our horse have no place that is bedded. They lay down on the ground, in the sand or on the grass or even on the mud. Sure they are dirty, but they have no trouble sleeping on the ground, all year long. Now, we don't get a ton of snow, but even during the snow weather, they find a place to sleep.
It is definiitel the shavings. Clean them all out, down to the matts and put a pile outdoors somewhere and see if she doesnt' change her habit.