Did you know that the maximum rate of fat oxidation during exercise is about 0.7g/mim! That's 42g of fat during your 1 hr of exercise--So, in a week in which you worked-out 5x--you would oxidize or burn 210g of fat or about a half a pound.
Therefore, if you are experiencing radical weight loss after starting an exercise regime--it might not be based on fat loss, but a lot of water and/or muscle loss!
It would take about a month to burn 2lbs of fat via exercise!
...So, be patient. Stop allowing the media and ridiculous weight loss tv shows to convince you that you can or should be able loss 20lbs overnight.