If it was the latter....retching is not uncommon with bypass operations depending upon the type.
It is caused by compression of the diaphragm and lower oesophagus against a closed glottis(windpipe.)
I simply cannot bring anything up when I goe through the motions of vomitting. I can have a full stomach and still nothing comes up. I have even taken a glass of water and 'thrown up' imediately and nothing came back.