I had horrible stomach issues while on abx.
This is simple really, abx kill bacteria, ALL bacteria. They kill all the flora (good bacteria) in the gut. This is why probiotics are imperative to take while on abx. An over growth of bad bacteria causes gas and a host of other issues.
Eating yougurt is not an good option since #1, it does not have enough probiotics in it and #2 it's dairy. Dairy when having any stomach issues should be avoided.
Drink lot of lemon water and take a good quality cold probiotic and you should be ok
The world breaks every one and afterward many are strong in the broken places