If you are insulin resistant, but not fully type 2 diabetic you might go thru two operating modes for your body.
Mode 1, high insulin level, storing glycogen in muscles and liver. Usually for 2 days or so after a high carb day. Water is stored with the glycogen, a lot of it.
Mode 2, low insulin level, dumping water that was used to store glycogen. Usually after a day or two below 70 or so grams a day of carbs.
Do you have a high BMI? Did you dump water after a day or so of low carbs?
I'm no doc, and sure don't want to practice medicine w/out a license... but I know folks who's fasting glucose was spot on for 30 years, and then were given a glucose challenge test, and were immediately classified Type 2 diabetic.
Then again it could be stress...