What Causes Foul Vaginal Odor During Sexual Intercourse?

Yes my boyfriend and I have been together almost a year. Very close to the time we met I had developed a strong odor and white discharge during sex and it inched. I took some otc for bv and it went away. But it burns him to have sex on my period and now months later the odor and discharge is back. But its only during sex. It's extremely foul. But no itching. And its not a fishy smell just smells dirty. What could this be?