What Causes Yellowish And Odorless Vaginal Discharge?

I have a vaginal discharge that is on and off. It is odourless n sometimes yellowish. I have tested negative for all STDs. What could it be?? Iam 25 years of age. Am about 154ft tall and weigh 7okgs. I have in the past suffered from syphillis which was treated. I have had a PDI afteran arbortion 4 years ago which was also treated.