I found something similar too -- and I'm nowhere even near that thin.

I successfully lost 50 lbs a few years ago doing keto and brought myself to exactly the middle of normal BMI.

While I was losing the weight, keto was easy -- I always had energy (more than ever before), felt great, wasn't getting sick.

But once I hit my target weight... all of a sudden keto became miserable. I had brain fog all the time, except for about the window of 2-4 hrs after each meal when my digestion seemed to peak. I started getting colds all the time.

Once I switched back to eating carbs, I felt great.

My theory is that your body burns fat proportionally to the amount of fat you have... and that it's fine to fuel yourself until you just don't have enough fat anymore to keep it going to even things out between meals.

I was pretty bummed actually to discover that keto doesn't work for me anymore, when I'm already at a healthy weight. I just loved the constant energy. But for however my body's built, it's just not an option for me anymore.