I got stomach upset when I took TEA... (?)

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Tea especially those brewed from real leaves and the unflavored one, has lots of benefits. Im not a drinker of tea especially those unflavored. One time I took a brewed tea. After an hour or two, i got stomach upset and went to the comfort room three times till my stomach got empty. What happen? Does that mean tea has ingredient that I am intolerant? Should I stop drinking tea? What should I add up to make fresh teas taste a lil better?