Is there a warning when you feed it? It might be underweight, so you'd have to feed it way more than the required amount.
Immortal only prevents your horse from death, it can still lose health. Achilles Heel says that time and energy are not important anymore when your horse ages, but it can still lose health to being under-/overweight.

I don't have the formula, I just checked some other riding horse which is about the same height.
this one, since I assumed Ows horses have some healthy weight.

It looks like your horse is very underweight compared to this one which is the same horse at the same height.

Bear in mind, also, that even once you get your horse up to the correct weight, sometimes needing to feed the maximum food for several days, your horse will not immediately regain 100% health. Health will improve by a few percentage points with each awakening, assuming correct weight is maintained. A shortcut is to use a Black Orchid, to instantly regain health - but don't bother if your horse is still underweight, as the health will drop again if so.

Good luck with getting her back to 100%. It's a pain, but we've all been there - easily done. Shame it's with a horse that needs wins for its benefit, as poor health will make this nigh on impossible.

Thank you all for pitching in! I tend to work my horses after feeding them so I was not giving her the oats needed. Appreciate the advice! <3