There are a lot of reasons for getting digestive problems but I doubt it has anything to do with poor posture. Either it's what you're eating and sometimes it catches up to you, or you're not chewing enough, or you don't have the proper organisms intact in your intestinal tract to properly digest your food or you have food allergies or intolerances you don't know about (wheat and dairy are the two most common ones but there are a lot of them some people have problems with) or you have a health problem or you have a bad anxiety problem. Just a whole lot of reasons for this sort of thing, too many possibilities to pin down without some professional help probably.