she's a pretty girl.
it looks like a bacterial infection to me, as to it's cause i'd only be speculating, possiblly small trauma to the area or a parasite of some type. i would isolate the horse to a QT tank as soon as possible. i don't want to alarm you, but there is the possibility that it could rupture and if it does it would be better if it happened in the QT tank and not your display. i know you have a thread over on the org so i don't want to muddy the waters, they're far more experienced than me. just to be prepared, what med's (antibiotics) do you currently have on hand?