Here's a good link regarding yeast, now this article is talking about candida but can likely apply to other strains of yeast
In a nut shell, a definite answer is not always easy to come by in medicine since many factors can give different results. Anything from the make up of your intestinal flora, your diet, to medications you take can have different affects that may be causing bloating or excessive flatulance. To answer your question, yes it could be the yeast activity but also it may be an intolerance to the yeast itself, also the bacteria in your intestines may not like the "new" yeast introduced and may produce additional toxins. Or a combination of all this plus more.