Its the salt. When you eat sugar/carbs you have high insulin levels. Insulin causes retention of salt at the level of the kidneys. When you go into ketosis you have very little insulin to retain the salt. So eat a little salt and you will feel better very quickly. By the way this is by far the most common complaint for people when they initially go into ketosis.
Both. Dizziness or lightheadedness can accompany a deviation of your body from "normal." this can include a deviation of your blood ph as in ketosis. It can be a warning sign that, without intervention, the situation can worsen.