You did a water change, but if you weren't doing regular ones before that emergency one, the issue could have started there. Your readings now won't tell the story, but a look at your routines might.
Betta splendens, as an air breathing swamp fish can take a lot of abuse that it shouldn't ever have to, but chronically unchanged water will get them. I agree with
@Naughts that this sounds neurological. That can have its roots in the kidneys, as it can with humans. The kidneys can be affected by polluted water.
Neuro problems can also develop in pristine water. Some things just happen with no cause we can see.
I've never had a fancy Betta with long fins jump. The drag takes it out of most of them. If it's a targeted jump, they go still and take an S shape for a moment, before springing off the tension of the tail. There's no twitching - it's a sleek hunting move.