Fishy vaginal odor that won't go away!!!

I have never had a yeast infection or even odor problems until I became sexually active with my boyfriend. I'm on birth control so he finishes inside me often but I always go pee & wash out his semen with water & I take good care of my vagina but I don't understand the smell. I'm starting to think it's because of him. I asked my doctor for something about the odor three times & every time I think it's gone, it comes right back once we have sex. What's this about & what can I do about this???