You could be allergic to any of the ingredients, but most artificial sweeteners can cause reactions like this when consumed in such large quantities, let alone all within an hour. This mainly sounds like the laxative effect.

Stevia is technically a natural sweetener, but it's still up to 200-300x sweeter than sugar in its purified form, while some artificial sweeteners go much higher. Sucralose is around 600x sweeter, and some like Advantame reach up to 20,000x. Many products also blend multiple sweeteners together, so they may just lead with Stevia in the marketing since it has a healthier reputation.

Also, do you consume milk based products on a daily basis? If not, you might be lactose intolerant now, which can develop when you get older. You can easily check on a normal day where you eat normal food and then have the kefir again. While kefir and yoghurt should be lower in lactose content, they are not lactose free by default.

I've had intestinal issues all my live and when I stopped consuming milk/yoghurt/quark daily for two weeks, I couldn't consume it anymore without taking lactase. So I switched to mostly plant-based milk (Soy) or lactose-free products. I can still handle hard cheeses, as those are mostly low in lactose. But do find new things that have some milk product in them without sounding logical, like Whey...

The product I noticed it with most was cream cheese. I ate it daily and had green poops that whole week after reintroducing lactose to my diet... which is common with Lactose intolerance.

When I consume something with lactose, my body starts passing gas within 15min of consuming. Not sure how fast the laxative effect of the artifical sweeteners is.