If no one had a chance to see your CD key, and you're the only person who knows it, then there's no chance that someone else is using it - try to remeber if anyone had the chance to see the key and copy it...
Well, there is a chance that someone actually coded a good keygen that generated your key, but guys at Valve swore that the chance to generate an original CD key is astronomically small, but even at 1 to a billion, there's still a chance - I hope that hasn't happened here.
You should try and contact Valve support and see what they say.