Oh yeah! I forgot about the excuse thing. I think your belly condition is a good one. You can said "I'm not in the mood today" but you don't have to say "I'm not drinking anymore." Sounds so permanent and then it will likely give rise to rebuttals like, "You don't have a drinking problem!" You can say it is only making you sleepy these days or whatever.
The less of a deal you make out of it in your response, the less they will have to respond to.