The pH of prepared kava appears to be between 7 and 8, essentially neutral.
In the following picture, the top strip was dipped in prepared borogu from KWK, and the bottom strip was dipped in plain tap water. It's hard to see in the picture, but the kava strip is a very slightly darker green than the water one, meaning the kava is just very slightly more basic than water.
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So the reason for your stomach upset is not related to acidity. Kava can cause stomach upset if it is not filtered well enough so that there are particles and fibers in it, and if you drink too much I think there can also be some kind of neurological action that can cause nausea. Be sure you are using a good filter bag rated a maximum of 80 microns.