What can cause yellow vaginal discharge with a fishy odor?

Bacterial vaginosis: A fishy odor is most commonly caused by overgrowth of certain bacteria in the vagina. There are several possible causes and while it is not necessarily a sexually transmitted infection it could be. A diagnosis should be obtained by pelvic examination. There are characteristic findings on microscopic evaluation of the discharge. See your doctor for further evaluation.

BV: Sometimes disturbances to the natural bacteria in your vagina (douching, irregular bleeding, unprotected sex, etc) can allow abnl bacteria like Gardnerella to overgrow and cause a fishy odor. It's not sexually transmitted, but can be treated w/ Metronidazole(Flagyl)/Clindamycin/Boric Acid. See your GYN for a prescription and be examined for other potential causes. Nothing urgent/dangerous though.