All Test Negative - what else can it be?

I have been experiencing excessive vaginal discharge. It is thick and white (sometimes a pale yellow) I have had two pelvic exams. After the first one, it was treated as a bacterial infection and I was given a week of metronidazole antibiotic. That didn't work. After the second pelvis, it was treated as a fungal infection and I was put on three days of terconazol vaginal cream - which while on the excess discharge pretty much went away, but in the days following slowly increased again.

I had had exstensive lab work done and bacterial and fungal cultures came back negative as did all tests for STDs including chylamidia, gonnorhea, herpes, hepatitis, hiv and syphilis.

I am going to another gyno for another exam, but my wuestion is, what else can this be? With all negative test results, what else can cause this and what else could they possible test for?