Your comment on the cake incident made me LOL!
Weight gain can be a symptom of gallbladder disease, so that could be one reason for the massive loss. Plus also consider inflammation, which is connected to overweight. Perhaps removing the sick organ and your body being able to recover and heal lead to the loss.
Enjoy it and don't blow it! You're 9.5 pounds ahead, even with half a cake! Don't wait until you gain before you get back on track!! Then you'll be kicking yourself!