What is this odor (female) ?

Ok so, I'm a 23 year old married woman, and have been on birth control for about 3 years (Yaz). I have only slept with one person and that is my husband. We don't use condoms and we do not pull out; he ejaculates inside me. For the past year and a half, it seems, whenever we have sex and he ejaculates inside me, I get a weird, fishy, foul odor the next few days to follow coming from my vagina. It's almost like a mildew smell, and I feel so embarassed as if everyone around me could smell it too! It only comes though when we have sex, I do not get this odor out of the blue, and it happens when he ejaculates inside me. Could it just be the smell of semen that has been sitting in there? I really enjoy my sex life with my husband- love it! But I hate dreading the scent I know is going to follow the next few days. Help!