Your partner probably has bacterial vaginosis; trichomoniasis is also possible but less likely. The odor of BV or trich is typically described as fishy, not like feces (but smells are very subjective). Another typical aspect of your story is that the odor of BV or trich often is most noticed after sex. (Semen is alkaline, i.e. high pH, which makes the chemicals responsible for the odor more volatile--so they some out of solution in vaginal secretions and escape into the air. Other things that can cause vaginal odor with or without discharge are a foreign body in the vagina, especially a forgotten tampon; or, in rare circumstances, other infections or a fistula (abnormal passageway) between the vagina and rectum.
Your partner should see her gynecologist or other health care provider, such as your local STD or family planning clinic. Treatment is available and effective. If it is BV, you won't require examination or therapy; but if trichomoniasis, you also will need treatment. Trich is sexually transmitted, but BV is not (usually). However, either infection is more common in people at risk for STD--so your partner's health care provider should also test her for chlamydia and gonorrhea.
Good luck-- HHH, MD