She does have some arthritis. I could see it in her hocks. Navicular wouldn’t surprise me.
When she went lame, it seemed to be very acute on one side, sharper than an arthritis symptom. In addition to maybe working her a bit hard for a first time out, she either bruised or pulled something.
And you’re right. From here on out shouldn’t really need to be lunged.
I’ve forgotten why you didn’t use her when Becky was out? Was she lame then too? Navicular is something that can be handled with proper shoeing and no hard work - at least for a long time. Of course, arthritis can be too, and exercise is generally helpful there.