Just a note to add on, Navicular disease doesn't usually just occur suddenly, it's usually years, or months in the making before the first mild symptoms arrive. Also, it's one of the more treatable/curable "diseases" if caught sooner rather than later, and many can carry on quite well even with internal damage if you can correct the trim enough to alleviate the symptoms. In the beginning, there is rarely permanant damage present, it's a disease of time and poor trimming/shoeing or other care related issues.
An abscess can be notoriously hard to find, even by a vet, and can cause swelling in the leg. How long after that "last ride" did she come up lame? It could be a stone bruise that is brewing into an abscess. I wouldn't assume Navicular just yet, it doesn't "fit" the usual route if she's never been symtomatic before now.