What causes fishy foul smelling vaginal discharge odor?

Bacterial vaginosis : The most common symptom of BV is an abnormal off-white vaginal discharge (especially after vaginal intercourse) that may be accompanied by an unpleasant (usually fishy) smell. This malodorous discharge coats the walls of the vagina, and is usually without significant irritation, pain, or erythema (redness), although mild itching can sometimes occur. Treatment is with antibiotics.