you have bacteria in your intestines that participate in digestion. when you take antibiotics you change this bacteria, some species become dominant, others almost disappear. this changes how your digestion works. in some people this change can be dramatic and lead to change in weight, etc....
www.diabetes.org/research-and-practice/we-are-research-leaders/recent-advances/archive/methane-producing-bacteria-in.htmltake more probiotics please and better quality ones/higher doses.