Vaginal swab came back normal, so what could this smell be? what do the swabs test for?

[Isabella's mummy :)]

I had a foul sour smell and burning downstairs, so I went to the doctors. She said it looked like a yeast infection and gave me a pessary, that made no difference.

I got the results back from the swab she took from my cervix and it came back normal, what could this foul smell/burning be?
The doctor never said what she was testing for