What's the cause of smelly vaginal discharge after sex. had a vaginal swab done and tests came out negative for everything. negative for stds too?

Vaginitis : several types of vaginitis. Can cause this odor see MD

Bacterial vaginosis?: The most common cause of discharge with foul vaginal odor (or odor alone) is bacterial vaginosis, an overgrowth of mostly normal bacteria. As you have noticed, odor often is most noticible after sex. Trichomonas, an STD, can cause the same symptoms. Treatment with metronidazole (Flagyl®) usually is effective. Good luck!