What is wrong with my horse?

There's a few factors to take into consideration... how old is your horse? She may just be feeling her age and having a snooze and/or relieving herself of her aches and pains that will be more evident in cold weather. How cold is it? Wild horses migrate with the weather like birds... The have a herd to huddle up in and gain body warmth from each other... nature is harsh and wild horses tend to have hard hooves and thick coats due to conditions and natural selection (the weak die out and therefore only the strong survive)... Personally I would recommend using the same amount of canola oil instead of mineral oil. Or just plain old vegetable oil. She's lucky she has somewhere to get out of the weather if she wants, do your best to make it wind proof. How often is she laying down and getting up??? That can be a sign of irritation and colic. Keep an eye on her and give her extra hay than what you normally would. It'll keep her body temp up while she's digesting it. My horse just went thru the last winter unrugged and he lost condition... its inevitable and especially morso as they age, I know I would prefer him rugged and he's a baby... just 3. All the best for Tootsie Roll :) hurry up warmer weather!!!