Welllll, Bacterial Vaginosis is just a bacteria infection which causes really strong disgusting fishy odor, and abnormal discharge. You might be able to notice if she has discharge that is on her panties, or even during sex.
It is supposed to look thin and greyish in colour. She would need to get that checked out by her gyno or doctor, because treatment is in their hands. It's not a dangerous condition, but it can be gross. The PH level thing is kind of interesting, many women believe that his semen reacts with the ph level and makes it smell rancid for a few days after.
And i don't know whether to buy it or not, because if that's the case it would happen all the time and that doesn't seem like a happy ending. But did you know how freakin common this problem is? I just googled it right now, there are so many women who suffer from the odor thing lasting for two days after, and they don't think they have BV. In anyway there are products like Refresh, a type of gell she can use to get rid of that type of odor for when it does happen.
I'm not sure how the product is used, but if you just put it in the vagina, you might be able to disguise it as lube, so as to not hurt her feelings. You can get it at drug stores. You also might want to just be honest with her. At least with an infection you can get rid of the odor completely. It could be something else, like maybe her hygiene is poor, I am assuming you ejaculate in her then, so does she wash up in there afterwards?
Lots of men have major unpleasant odors due to what they eat, and perhaps you are making her smell like that, but if that's the case it shouldn't last for days, unless she doesn't bathe. You could just out and tell her, politely, say you are just worried she might have an infection. If you are worried about hurting her feelings you could put on an act and say you would like both of you to just go get checked at a "sex clinic" not sure what they are called there for a random reason, and hopefully the doctor will see if she has Bacterial Vaginosis.
I'm hoping she would notice if she has a bacterial infection, if you are noticing it, chances are the smell is more than obvious to her, and if she has abnormal discharge, she hopefully will do something about it. If not, it would be best to tell her. Or if she doesn't notice anythings wrong, maybe it's just the ph thing, and next time you try and have sex with her, and the stench occurs, bring her into the bathroom and have sex in a shower, or bathtub, but actually try and clean her in a sneaky way.
Or use the Refresh, haha.