There are a lot of things that could've caused the little weight gain. (Note that it's a weight gain, but probably not a
fat gain--so don't get too frustrated at it.

) Sodium intake, "That Time," a heavy meal that you haven't yet fully digested and excreted, not drinking enough water, drinking too much water and not having peed yet, weighing with wet hair...really, there are a LOT of things that could make the scale register an extra pound.
Look at it as a good reason to examine what you're doing rather than as a sure sign that you've done something way off your plan, because chances are about 98% you didn't.
Have you changed your eating habits at all since you stopped drinking the bubbly stuff? Some people do a little compensatory nibbling when they first quit something they're used to having (I know, I gained 25 pounds or so after quitting smoking). If you keep a food journal (and I hope you do because it's incredibly helpful for spotting trends in your weight loss), look it over and see if you've been eating a little extra.
Or it may just be one of those weird things bodies do. Weight loss isn't linear. The pound you saw may have been from some indulgences a week and a half ago or longer. We have a natural tendency to look at the stuff we've done in the last few days for hints as to why weight loss has slowed or sped up or plateaued, but really, it can be a matter of weeks before something shows up.