It could very well be an intolerance. Soda is just like liquid candy - and if your system hasn't had any for a while and then gets something into your bloodstream as fast as liquid candy will, it could affect you just that quickly.
I, otoh, had noticed something about this WOE. Since starting Atkins, I have had very little intestinal gas. Very little - especially compared to before I started Atkins. I was a gasbag!
Had my cookie dough and resulting tummy ache this weekend and then followed that up with lots of gas.
Back to square one this week - strict induction with no nuts and so far, no gas. I plan to stay on induction without variance for several weeks/months now.... now i know I can't do nuts.
It's an odd phenomenon, but I'll take it. My insides are happier without the carbs, too.