Re: I stopped Nexium and...
It may be the "rebound" effect, often reported by people here who stop cold-turkey.
It's not always very worrisome. Some people simply have their stomachs' "pumps" (as in PPI) start working again by dropping the PPI and the GI system is no longer used to much acid being around. This can cause irritation, and I'm sure gas is possible, too.
I would just ride it out and see if you feel better a week or so later. Many say that they'll get the "rebound", but if they wait it'll go away.