If I cosume only 700calories/day will it affect my health?

As far as the "I can regain the muscle later" comment-- it's a lot harder to build muscle than I think you realize. It's much easier (and better IMO) to hold on to the muscle you have while you lose fat.

I have did this and lost 24 lbs since 1/1/13, but I have no energy (I LOVE lifting weights). I am supposed to eat 1860 calories a day, but lowered it to 1700 with 400-650 a day of aerobics. I suffered the next day when I realized I ate only 1200 calories the previous day and did 500+ calories of exercise.

Instead of going to the gym or the 2nd 40 minute walk with the Lab at home, I dozed off on the couch after work. By 8pm I was in bed.

I've been losing about 1kg a week for the last 2 months n I feel grate. But I've just realised I havent had a period for the last 2 months, im definitely not pregnant so could this b down to my diet? I average about 700 calories a day.

Problems with extremely low calorie intake (most of this stuff doesn't happen right away, but later down the road...its kind of like how some smokers end up with emphysema or lung cancer and some don't):

- Malnutrition

"Inadequate caloric intake can lead to malnutrition over time. According to MedlinePlus, malnutrition occurs when the diet does not allow the intake of a sufficient amount of nutrients. A diet with extremely low calories does not allow for adequate consumption of the vitamins, minerals and antioxidants the body needs to perform its daily life processes. Vitamin deficiencies can lead to various problems, including fatigue, impaired wound healing and extreme weight loss." *1*

- Slower Metabolism (takes a while, but does happen)

"Metabolism is the rate at which the body burns fuel for energy. The basal metabolic rate is the minimum number of calories necessary to perform daily basic functions such as breathing, growth, cellular repair and digestion, according to MayoClinic.com. When calories are restricted over a long period of time, the body will compensate by holding on to the few calories that are consumed. Instead of burning calories freely, the body will perform basic life processes more slowly to conserve fuel. This can actually lead to weight gain." *1*

- Physical Changes

"A decrease in caloric intake may cause positive physical changes at first, especially in individuals with extra weight to lose. However, an extreme decrease in caloric intake can cause negative physical changes over time. Once individuals lose excess weight, calorie restriction can cause various physical problems. The Calorie Restriction Society indicates that decreasing daily calories too much can lead to reduced bone mass, fatigue, sensitivity to cold temperatures and food obsession due to extreme hunger. The CRS recommends gradual calorie reduction over a period of at least six months to avoid some of the side effects of low caloric intake." *1*

-Side Effects:

"Let’s look at some of the uncomfortable side effects of very low calorie diets:

Dry mouth
Headache
Dizziness
Fatigue and weakness
Cold intolerance
Dry skin and nails
Menstrual irregularities in women
Hair loss
Constipation/diarrhoea
Irritability and confusion
Inability to concentrate
Muscle breakdown
Problems with nerve and muscle function due to an imbalance in minerals and electrolytes
Conditions such as osteoporosis, anaemia, gout, gallstones, clinical depression, heart problems, renal failure, and liver disease" *2*



References:
*1*
http://www.livestrong.com/article/277053-what-are-the-dangers-of-low-caloric-intake/
*2*
http://www.dietriffic.com/2009/05/14/very-low-calorie-diet/