It could well be that the low-carb diet is masking the diabetes - you are already lowering your blood glucose levels with your diet.
It is usually recommended that a low-carber should eat at least 150g carb per day for 3 days before a GTT. That way you get a more "normal" reading. After all, all that diabetes is, is an inability to metabolize carbohydrate correctly, for whatever reason. If you're low-carbing, it doesn't show.
I low-carb too, and my levels are now hardly ever out of the non-diabetic range (except when I have a blip!

) But I'm still diabetic, and those blips, eg about 3 days over Christmas, let me know that I am.
Don't worry about it too much - if you're going to low-carb for the rest of your life, then you are already on the way to controllng Type 2 diabetes if you do have it.
Viv 8)
PS Contrary to what the press like to tell us, being fat is not essential to being Type 2 diabetic. In fact, about 20% of Type 2s are skinny. Just as about 80% of overweight people are not diabetic.