My suggestion would be to either not go to the party, or just drink soda/water if you do. You have no obligation to tell anyone anything about why you aren't drinking. Trying to cover it up by lying and drinking something that resembles alcohol is not going to help anyone.
So if you don't feel you can handle the social situation, just avoid it. And if you do, just don't drink. You should be much more concerned about your own sobriety and the potentilal to slip up and drink yourself than what other people think about you.