I would NOT administer dex unless you have a have a definitive diagnosis of something that requires it.
Dex runs a small risk of causing laminitis in some horses, which is not worth it.
Is she a Peruvian Paso? She looks like some kind of Paso. If so, they are susceptible to developing DSLD, a suspensory ligament degeneration that shows up in the hind legs first. There would be chronic subtle lameness and swelling in the area you pointed to above the fetlock, in the lower branches of the suspensory ligament just above where they connect to the sesamoid bones at the back of the fetlock.
But the history points more toward a foot abscess, with subtle lameness for a day or two then sudden non weight bearing.
Hoof abscess will cause a definite increased digital pulse, but a damaged suspensory ligament would not, if that helps.