Your body is going through Ketosis...It is a good thing.  It happens with rapid weightloss.  Urine gets darker and you may see the symtoms below. The other symtom that is normal is the strange taste in your mouth which taste like lighter fluid...because it really is lighter fluid...acetone being released.
What is Ketosis?

Ketosis is the name for a state many people go through when starting a low-carbohydrate diet. In involves a loss of fat and it happens because, in the absence of sugar, the body will break down fat to use as fuel.

This fuel is known as ketones, and the process of creating ketones in the liver and the body's use of them is called ketosis.

Understanding Ketosis Symptoms

You can actually develop ketosis symptoms that will let you know this is happening in your body. For a lot of people who begin a low-carb diet, this seems to happen three or four days into the process.

Some ketosis symptoms include:

• Tiredness or fatigue
• Headache
• Feeling thirsty all the time
• Bad breath
• Metallic taste in the mouth
• Weakness
• Dizziness
• Nausea or stomach ache
• Sleep problems

Ketosis symptoms usually last for several days—up to a week in some people—until the body is used to burning fat instead of glucose. The levels of ketones in the body (which can be measure through a urine test) will go down, but that does not mean that you are no longer losing weight. It simply means that your body has found balance and is no longer producing excess ketones.

Sometimes low-carb dieters never seem to reach ketosis, or at least never have the urine test strips indicate excess ketones (the strip turns purple when there are excess ketones). This could be because exercising has used up the excess ketones or the urine is diluted from drinking a lot of water. This doesn't mean these people aren't burning fat, just that they aren't registering as technically being in ketosis.