Lately every month or 2 , i vomit during a bm. don't feel sick, but food all suddenly comes up, then i belch a lot, while defecating.episodes always preceded by eating & sometimes by mild constipation.

Constipation: I recomment a trial of stool softners, eating small meals but more frequently to provide sufficient nutrient, and eating a lot of vegetables and fruit. If this does not work, please see a gastroenterologist.

See a GI doctor: And do this quickly. Although this may be quite benign and represent irritable bowel there are many potential causes which can be severe and this needs a detailed investigation. Go asap and get this diagnosed and treated. Good luck and best wishes.