Well, my other answer is that ketostix are notoriously unreliable. A lot of factors can lighten the color indicator. Exercise is actually one of them, since you are using up the ketones as fuel. Not sure why your readings changed so suddenly, but it could be so many things that it's probably not worth worrying about. My guess is that you're still burning fat, it's just not showing up as strongly. In my eyes, a trace is enought to prove the existence of ketones, thus the process is in effect.

Forget the dumb sticks. And keep up the good work, that's quite an exercise regimen. Make sure you get some good sleep in there as well.