I asked a friend of mine about this - she's a fitness and nutrition consultant, and she said she had a great online article link about this - I copied some of it here but here is the link if you want to read more:
http://www.goalguide.com/ketosis.htm

Ketosis happens during fasting, low carbohydrate diets, pregnancy and it can be caused by diabetes. You use up your store of glycogen (blood sugar) and then your body permanently goes into fat burning mode to fuel your bodily functions. This change in metabolism occurs when the body no longer has incoming carbs to produce glycogen and after the liver has been depleted of stored glycogen. When either stored body fat or incoming dietary fat is used as fuel it breaks down into ketones.

Ketones can be used as fuel for your body or discarded via your breath or urination. It is the rise in ketone production that means you are in ketosis. Take note that although you are in ketosis this does not mean that you are *only* burning bodyfat. The released ketones may be from either bodyfat or dietary fat.

What are side effects of Ketosis?

Bad breath - which is due to ketones being released through your breath, drinking more water will lessen this effect. Some people also report that they get a metal taste in their mouth.

Strong urine odor - due to the presence of ketones.

Light headedness or "foggy" thinking. This does not occur in everyone and many who do feel this often are helped by making sure they are getting enough to eat, enough water and taking potassium supplements.