How can I lose 10-15 or even 20 pounds in a month?

The healthy amount of weight to lose in one week is up to 3lbs, so if you lost 3lbs every week for a month you'd lose about 12lbs, but you can lose more. You might have a slow metabolism, which you can get pills for. Since you already eat healthy and work out, that's the only thing I can think of.