Cum sometimes makes my stomach hurt

Most of the time I'm fine with swallowing but every here and there, I my stomach will hurt after swallowing or I'll get reflux but I can't figure out if it's the cum itself or other contributing factors.

Does this happen to anyone else? I can't remember the other times it's happened but last night was my first time with this particular guy and I swallowed on kind of an empty stomach... I had a snack prior just to settle my stomach because I was staving.