Hello, There could be many reasons.... Although not a sexually transmitted disease it can be yransmitted to you by your partner There could be STI like trichomoniasis or gonorrhoea or bacterial vaginosis Use of birthcontol pills,estrogens,excessive antibiotics Associated diabetes Use of synthetic clothes and frequent douching Your yeast infection could be due to some other species of candida like C GLABRATA which responds to nystatin and not diflucan Consult a doctor and get the microscopic examination of vaginal discharge done and get properly managed. Thanks