she may have a yeast or bacterial infection which can happen naturally, has nothing to do with her sexual partner. this could also just be the way she is now, especially if you were her first sexual partner. also, have you been tested for std's or are you just assuming you have none? because std's can be dormant for some time and all of a sudden show up, yes even up to a year or more later, particularly if she is under a lot of stress now. i am unfamiliar with an std that causes odor. she probably has a yeast infection. in any case, you should have tests done too.
Just because she has a vaginal odor does not mean that she has an STD. If this smell is out of the ordinary for her then she may have some sort of infection. Not all vaginal infections are sexually transmitted, they just happen.