I don't eat carbs when I start eating again for the same reason. Try eating per the Atkin's diet for a few days and gradually add carbs back in. As an aside, I've used the Atkin's diet to lower my cholesterol and the "keto-strips" indicate a light ketosis after 3 days or so.
I do IF (Intermittent Fasting, 24 to 36 hours with water only once a week), and have found that too, but I have also found that as soon as I start eating again, I get quickly tired and warm (like I started a fire inside my body).