Is Cola good for an upset stomach?

Not really. The best for an upset stomach would be Ginger-ale. Ginger has been proven to ease nausea. That is why most people recommend Ginger-Ale, because of the ginger. Try to get something like Canada-Dry (available in some northern states) or another brand with real ginger. Stay away from the simulated ginger crap. That will make you feel worse.