The gas in the fizzy drinks are possibly just giving you really bad wind. I woke up the other day in agony - and all i needed was a nice big fart and i was sitting pretty!

Going from a fairly small amount to a fairly large amount will take it's toll - but your body will adjust over a couple of weeks.

Try slowing the pace down and see if this makes a differnce - if not, go back to the normal pace and try cutting out the stuff with gas in it.