There is a condition called fish odor syndrome. If you google TMAU you can read more about it. TMAU is some kind of enzyme condition connected to the liver. I can never remember the correct name but TMAU will bring you to the right websites.
Sometimes this could be caused by a yeast infection. It can also be caused by vitamins. Certains foods could be culprits too like the other poster stated. Try eating yogurt on a regular basis. Have you asked your doctor about this? Drink alot of water too.