If you know for certain that you aren't swallowing air (some people do this without knowing they are doing it), then it must be gasses produced by fermentation somewhere in your digestive system. Fermentation is the process whereby some bacteria and/or yeasts produce energy in an environment deprived of oxygen (anaerobic). If you produce a lot of belching some time after taking antifungals then I would assume it is yeast (die-off). The longer it takes you to start belching after taking antifungals, the lower down in the gut the yeast are. Of course even if you take no antifungals the yeast will produce gasses (CO2) on their own. Some people with SIBO also have bloating and gas. I am not a SIBO expert. I didn't have that problem. If you need help with SIBO do your own research or ask someone else. The solution for the belching problem is of course to solve and get rid of the yeast. Read all of my posts as well as dvjorge.