It can be difficult to talk to partners about personal odor, especially if they think that their smell is "normal." Have you considered the possibility that this is her normal smell? If you are sure that it is not, then proceed with caution.

If you truly believe that something is abnormal, the most way to be most sensitive to her feelings might be to bring a trusted mutual girlfriend into the mix. If a friend pretends to "happen to notice" that she is "not so fresh" she might be more receptive and less embarrassed that if you tell her. This can be difficult to organize though, so speaking to her directly is probably the best plan.

You might start by "noticing" that she smells "different" than usual.
Pretend that it's a new thing. Tell her that her smell has changed and that she used to smell amazing but that lately it seems a little different (but still the best smelling vagina you've ever been around) This might be best received after a really enthusiastic round of intercourse or (even better) oral sex when she is in post-orgasmic bliss.

Offer to go with her to a clinic. Tell her that you've heard that new partners, new sexual practices, or condoms/lube can sometimes affect vaginal chemical balance and that you want to make sure that your sex life isn't negatively affecting her. While you are at the clinic, you can both be tested for STIs. It is possible that she has an STI she is unaware of. When you are both tested, she may test positive for bacterial vaginosis or trichomoniasis (or something else.) Your presence with her at the clinic will demonstrate your love and support, and you can probably avoid getting punched in the face.

planned parenthood is a free, low cost, or sliding scale clinic that has locations all over the US. To find one near you check out http://www.plannedparenthood.org/health-center