It’s pretty normal for this to happen during a fast. The green tea could be a factor. Mix a teaspoon of baking soda into a small amount of water (just enough for one gulp) and chug it back fast. Swish and follow with some plain water afterwards. Tastes awful but it should get rid of the indigestion immediately.
It’s been my experience that you could get acid reflux suddenly and for no apparent reason. I don’t believe it’s anything to worry about; it’s just annoying. A sugar-free antacid should help if you feel uncomfortable.