Had something similar happen, but maybe not quite? I was doing DATY and I think she started to get wet, and the wetter she got, the more it smelled like a can of tuna, I literally had to stop and put my head up cause I thought I was gonna puke all over her. I've never encountered a smell like that before, and it wasn't even overpowering...it was just such a vile smell that I gagged. Naturally, I didn't say anything and moved on to something else that didn't involve my face in that general vicinity. It's strange, cause it's not like she smelled when I started, but once she got wet, that's when it hit me.
I sometimes wonder if maybe some purposely let it get a little rank down there to discourage the DATY, and they know that most guys won't say anything. I mean, I know when I smell, so I'd imagine an SP who's livelihood is basically dependent on their body should know.
That sounds like bacterial vaginosis, and it is possible to not know you have one until its past the not noticing stage. Yeast infections generally don't smell, but BV definitely does. It starts off not smelling, and then as the infection gets further along, it can become quite rancid. In the beginning stages, it only smells when you get wet. By the time, most girls notice, she should have gone to the doctor all ready. The thing with BV and Yeast infections, is most women/girls don't know they exist until they get one, so they don't know what the symptoms are. I do a finger test on a regular basis, hahaha. Once you get one though, you know what to look out for.
They are not std's. They are both caused by a ph imbalance. One is caused when you have to much, the other when you don't have enough. Certain condoms, soaps, baby wipes, and douching will make both of them even worse. Which is why sometimes they get so bad. The woman thinks she's trying to fix it, but bandaids it for a few hours, then its worse than before. Yeats infections are cured with over the counter medicine that yu can insert. BV is cured by a trip to the doctors, because you have to take a pill. Some women, ph balance can be upset really easily. Non perfumed soap, the right kind of lube, eating yogurt, and paying attention to the kind of condoms used, clean fingers and nails for digits or not allowing digits, can make a difference.
Man, its hard to be a girl

I'd rather you tell me, then read about it in a review, hahaha. How embarrassing would that be, lol. You could just say, I think you may have gotten a bit of bv. There are polite ways to bring things up, lol