What is the most common cause of fishy vaginal discharge? i have been in flagyl several times for bv and nothing works. what else could it be? could i've cancer?

Probably still BV: Probably you still have BV, which is often difficult to treat, not responding to usual treatments or symptoms returning soon after treatment. Avoid sex or use condoms consistently for 2-3 mo. Cancer is not likely. Keep working with your doctor, or see a Gyn who specializes in infecitons. Alternate treatments for BV include clindaymycin, boric acid vaginal capsueles, and others. Good luck!