Yellow clumpy vaginal discharge sometimes clear, no odor, no itch, negative for c&s test, wet mount and pap test. What else can cause yellow discharge?

Treat for best guess. Diagnosis of vaginal discharge can be difficult. Yellow color suggests infection. Depending on exact tests done, some infections can be missed, e.g. gonorrhea, chlamydia, trichomonas. Talk to your doctor about being retested. If available, include the new DNA test for trich, much more accurate than wet mount. It may be necessary to treat for the best guess, without proof of diagnosis. Good luck!
Probably normal. What you describe is classic for a yeast infection, but that almost always causes major itching! Bacterial vaginosis discharge is typically grayish, cloudy & fishy smelling. Trichomoniasis is heavy, yellow/green, with odor. Gonorrhea or chlamydia is thick, yellowish/greenish, with discomfort. Yours is likely normal-the result of fluid produced by glands lining the vagina and cervix. OK to observe.