Probiotics. Fermented foods, such as unpasteurized sauerkraut, kimchi, etc.
Start lightly and add more probiotics as your gut adapts to its new 'environment'.
Also, do you eat dairy? Gluten? Grains? Legumes?
Might wanna check if you are intolerant to any of these, as well as FODMAPs.
Maybe try and eliminate each of these for a few weeks, one at a time. Check if you get better.
I'm sure you can get tested for intolerances pretty easily, too.