What Would Cause Someone To Have A Bowel Movement And

What would cause someone to have a bowel movement and after going to the bathroom, start having saliva build up and then forceful vomiting at the same time when they are not sick? I have had this happen a few times a year, but lately it has happened 2-3 times in a month or so. I do not ever feel sick, before or after, but have such force behind the vomiting that I feel slightly weak for about 20 minutes after my episode. I have had several abdominal surgeries in the past, could this be from adhesion build up?