It is 3500 calories equals a pound, whether it is eating that much over - and you gain one pound - or eating that much less - and you lose one pound.
However, anytime I have started a new eating plan I always lose the most weight in the first week or two. So losing 10 pounds in 11 days isn't unusual at the very start, but you weight loss should level out and more like 2 pounds a week from here on out. Unfortunately our bodies are not calculators and can be affected by stress, hormone levels, fatigue, etc., so we will not always see what we expect to see.
My advice would be to keep it up, and see what happens as your body adjusts to these changes.