Why Am I So Bloated After A Small Snack?

I'm on Atkins Induction, Day 6 (if that helps any), and all I'd eaten before now were some eggs and half a cheese stick, but that had no effect on me. I had some Sprite Zero and that didn't affect me either. However, I just had about 1/3 of a Mountain Dew Voltage Diet soda and a whole cheese stick, and I bloated up like crazy. Is there any reason why this happened?