Definitely worth a chat with your/a farrier. Also consider a chiropractic visit (for diagnostics now, and possibly future maintenance) as the mare is apparently moving differently than priour to this and it might be a misalignment within the spine causing her to pull her hind feet under her for support. The single-tracking or crossing over of her hind feet implies a hind end issue but not exclusively. The health and integrity (or lack of) of hocks, stifles and hips/pelvis can generate ways of going that can produce underrun heels in back..(because of her age, I might be inclined to rule out arthritis). Shoes MIGHT not be in her best interest AT THIS TIME as even a well placed shoe can change the horse’s WOG and force further compensation if there are any existing issues. Determine the cause first, then fix from the inside, out.
Just out of curiosity, stand her square up. Look from directly behind her. Is she standing even and symmetrically developed on both sides? Hips/hocks level? Tailbone perpendicular to the ground? From the front, shoulders/elbows/knees even? Facial midline perpendicular to the ground? Ears level? Barrel even on both sides? Equal weight bearing on all four feet at the same time? Looking from BOTH sides, is she smooth along the top line? Does she fuss/shift (beyond balancing) at all about one foot or another when you pick hooves? Just the process of squaring her up might shed some light for you.
Also and equally possible: she is favouring her front end for some reason (pulling her hind under to lighten the front), the reasons being similar to already mentioned. Something as simple a a slip in the mud, got up wrong from rolling or even just fell or maybe just torque’d herself in the paddock… any of these occurrences can create a wide variety of injuries from imperceptible to outright Home-worthy. One wrong step can set off a domino reaction faster than a set of… dominoes! Horses can hurt themselves so many different ways and to such a wide degree of offense! And some (too many) of these physical insults can be ohso subtle.
I would venture to say that you might be seeing the result of some injurious (however minour at the time) action that had happened a while ago, the underrun heels in back being the result of compensatoury movement. Good luck with this situation, I do hope you find a cause and solution soon and can get back to a happy mare : )
Horses. They are so fragile yet engage so completely in such freight train moments…
Fingers crossed for both of you, JR!!
Sure there's right and wrong but mostly there's just a whole lotta different.