You didn't do anything wrong. You should probably stop adding the drops until the stomach pains are gone. Then be more accurate next time with at most 2 drops per liter. I'm not really sure how much as the 28% is much stronger.
I just put one drop in the top and bottom of my Berkey water filter to kill any bacteria that may have developed. Then I put in 3 pitchers of tap water in the top. I hope it doesn't give me a stomach ache.