Odor question???

I never have an odor and never an odor when masturbating and I shower daily, but when my SO and I are together intimately I have kinda of a fishy odor. That's the ONLY time I ever smell it. No discharge, no smell any other time, no itch or anything indicating an infection. Please help, what could this be? I noticed the odor after the third time of sexy sexy time and just hasn't seemed to leave after that and like I said, it's ONLY when having sex...it's so embarrassing and I feel I can't enjoy sex because of this problem. WHY and WHAT COULD IT BE?