Please help

I was told today at my OB appointment that I have ketone in my urine. No protein. I do have gestational diabetes that is under control. I’m On a no carb diet. No high blood pressure. I Drink 18 bottles of water a day. My OB wasn’t sure as to why I have that in my urine.

Could someone please tell me why would there be ketone in my urine?

Google doesn’t help much.

Sorry this is a bit choppy, I just want to be to the point.