Stomach problems after sex

I am a 21 years old male and I have stomach problems after sexual relations (may it be intercourse or masturbation). A day or two after sex (usually the day after), I have loose stools and sometimes diarrhea for a few hours and then everything gets back to normal. Also, when having sex with my girlfriend, she sometimes happens to have the same problem as me. What could it be related to?

[alexin7]

Might you have a digestive yeast problem? This would be an unusual problem (that I've never heard of before), but could it be that the semen or other sexual residue you leave on yourself after sex contributes to yeast infection in digestion? If that is the case, it may help to wash your penis sooner and more completely after sex.
Do you think that could be it?

Or another thing, is there pain that you can feel move when you cough? That would probably be a hernia, for which you'd need to see a doctor asap.