first of all, keep it up!
8 pounds in 3 weeks doesn't sound unhealthy or too extreme. if you were dropping 10 pounds a week, you would have good reason to be concerned.

second; are you eating right or watching what you eat at all? are you going on longer rides, or just commuting? basically, is there anything else you've changed since october 1 that would cause weight loss or gain?

also, each pound you lose is harder to lose than the pound before it. the first pounds are the easiest to lose, and you have to work hardest for the last. you will eventually get to a point where the energy you're putting out via commuting isn't enough to cause any more weight loss.