One of the possible solutions might be to try decaf tea. Although decaffeination also reduces the amount of polyphenols to some extent, in many cases there is still a lot of them left. The reduction of polyphenols seems to vary among different brands according to this article so it might be worthwile to try several brands before you come to a conclusion.
Another approach might be to try tea with milk, since milk proteins precipitate tannins. However, this reaction is pH dependant and works better with other types of polyphenols than those found in tea, as shown in this research, but if you compare it with how you reacted to decaf tea, you might come to a conclusion.
It is important to stress that your stomach ache, even though exacerbated by tea, might have a different underlying cause. If it occurs regularly with tea drinking or if it is not regular but causes great discomfort, you should talk to your doctor to get to the bottom of the problem and receive adequate treatment.
The Bioactive Compounds of Tea and Decaffeinated Tea (Camellia sinensis)
Interaction between some plants tannins and milk protein