What can cause yellow discharge and odor, negative on STD test, and just got treated on yeast infection?

Vaginitis. You probably have a bacterial vaginal infection. This would not respond to treatment for yeast and would be negative on STD test. See your doctor, antibiotic would help this.
Bacterial vaginosis. Bacterial vaginosis (BV) is the most common cause for a yellow vaginal discharge with odor. BV is not an STD.
Bacterial Vaginosis. Any change in the normal flora in the vagina can affect the smell, colour, or texture of the discharge. However with a negative STD results it is probably more than likely Bacterial Vaginosis. Bacterial Vaginosis is a bacterial infection that is not sexually transmitted.