Hi Juniper. Since the kidney problem is a new problem it's not likely you have done anything permanent to it. Just keep up the water like you have been doing and maybe try the Watermelon flush a couple more times. It sounds like you handled it well once the symptoms started.
It did sound like your blood Sugar was way low. I can't ever do a Water Fast because of my blood sugar. If your going to insist on doing a fast try a juice one. Preferably freshly juiced. Make sure to sip the juice all day long that way for at least while you are awake you should stay stable. When you go to sleep set a small amount of juice by your night table just in case you wake up with that low blood Sugar feeling.