Just tell the truth. If he is weired out, who cares. Still go with him and socialize or whatever you guys are doing.
I usually tell others the truth, they get weired out for a moment, then it is forgotten.
But I do understand that feeling. Also that feeling of being with others for a large part of the day, and not bringing your lemonade with you because 1. it's too difficult and 2. people would be wondering what in the world you are doing, and even if you did explain they still wouldn't really get it.
If that happens I just drink a whole bunch of water, come home and double up on my lemonade drinks.