In my experience as a non-drinker in a drinking situation, I have found that I end up being more concerned about it than everyone else is. The situation that I was most uncomfortable with was going to an open bar wedding reception where a fellow guest (who we ended up sitting at the same table with) was a guy I used to drink heavily with. He didn't even ask why I was drinking soda instead of pounding whiskey.
If anyone does ask or says anything, all it takes is "I'm just not drinking tonight". 100% true, no excuses or fibs needed. You don't owe anybody anything. If they pry beyond that, end the conversation.
SC