1) You burn more energy than you consume (you exercise like crazy)
2) Brain: "Whoa whoa whoa, we don't have enough energy for this s**t. Use something from our fat storage.
Body: "Okay boss, let's burn some fat."
3) Brain: "Ok, this fat ass doesn't eat enough, we need to save up some energy. Lower energy consumption of organs."
So you lost some weight. Now there is a problem. Your energy consumption is lower than before losing weight. You need to keep exercising and don't eat too much or you will gain weight pretty fast again.