Fish smell after sex

I get a fish smell every time I have unprotected sex with my oh. It will last 48 hours or so and washing does not get rid of the smell. It always goes away within 48 hours but does anyone know why this is? There's no itching & just a slight watery discharge. Any ideas on what I can do to stop it happening? I wanna wear shorts today but scared of some one smelling me!! Lol help