Angie,
I think that it probably is your diet, at least that's my guess :p . Out of curiosity, are you eating primarily protein, or are you also eating a good amt. of carbs as well (I know you had pasta last nite, but in general do you eat carbs?). B/c the whole Atkins, and many other fad diets, are based on just eating protein...which I'm sure you know already. But the goal in those diets is to produce ketones..that's a way to show you are burning up your fat stores. However for diabetics, it's not such a safe things to do. Even if you aren't doing the hi protein diet, it could be just that you are still burning fat, hence the ketones.
The only other thing that comes to mind is that you may be getting sick. I think I have read that illness that leads to dehydration and that kind of thing can bring on ketones. It doesn't sound like you are feeling ill, so this probably isn't the reason.
If this continues, I would contact your doctor (just to be safe). I'm sure he or she will provide you with more insight than most likely we can.
Keep us posted
