<human>: Could you distill the essence of the text into 1-3 sentences?

I have been dating a great girl for the past 1.5 years. Things are pretty good. She is always down for sexy times, which is totally and completely awesome. But one problem has reared its head. When we are having sex doggystyle, I can't help but be overwhelmed by an incredibly pungent (and at times rancid) smell emanating from her rear. It smells very poopy. 

When the smell hits, I lose all interest and usually just stop having sex claiming to be exhausted or something. It's a huge turn off. In the immediate aftermath, I generally find myself disgusted and frustrated, as if she does not know how to bathe properly. After a little while the feelings subside and I get back to normal. 

I am uncertain whether or how to say something, or whether it could be some condition that I don't know about. She is a brilliant, smart girl, but she also isn't the most girly. Not that it's a telling sign, but women I have dated in the past usually have loads of different soaps and shampoos lining their showers and a multitude of lotions to fit their mood. She does not. 

Is there any way to lightly address this with her? Would it be better to do it in the moment or to wait until a later time when we are not having sex? I think this will really embarrass her, but I feel it has to be addressed.
<bot>: Noticed very rancid smell while having sex with GF from behind. Has happened a few times. Worried about her general hygiene. Want to say something, but don't know how or when.