Since it is a fishy smell, it is more likely bacterial vaginosis than yeast. If it is yeast, it would probably itch, and be swollen. Hopefully, it is not an STD, but if it is, from what I have read, the only one that has a fishy smell is trichomoniasis, and that is not a constant fishy odor.
When you got your pill, did you get a gynecological exam? BV (bacterial vaginosis) can be passed on by doctor's equipment, if it is not clean, as fresh discharge can be contagious. Or as DawnWine says, through an imbalance of hormones, or a pH imbalance. It would probably have to be cured by Antibiotics or other prescription medicine. Many of us here have tried "natural" cures, which work for awhile, then it comes back.