weight loss to fast??

Once someone switches from eating fast food and junk food to just eating healthy nutritious things, they notice a different almost right away. Not to mention the reduction in calories you were eating before and what you’re eating now. Women tend to lose slower than men because men are larger and have more muscle than women due to the hormone testosterone. They are genetically designed to have a higher percentage of muscle and less fat -- which works in favor of keeping them fit and allowing them to eat more calories. I hope that helps.

Not sure if you had a question about fasting in there somewhere.

If you're considered heavy already, losing that much weight isn't uncommon at the beginning of your change of lifestyle/working out/eating healthy. If you're fairly skinny and drop 20 lbs in a week or so, then you're either on the Biggest Loser or someone harvested your kidney and liver and there's cause for concern.

Fasting isn't a huge issue if it's very, very short term. But, if you don't practice healthy eating habits after then you're going to fall into your old ways.

Read long term issues of low calorie eating:
http://www.myfitnesspal.com/topics/show/3047-700-calories-a-day-and-not-losing

If you're eating better, there is a chance you're consuming considerably less sodium as well, so a portion of your loss is likely water weight. So I wouldn't sweat the 20 lb drop unless you continue to drop at this rate, in which case maybe consult a physician .