Is a fishy smell only on your period normal or does it indicate an infection such as bacterial vaginosis?

Could be BV: It is possible to have an overgrowth of gardnerella (causative bacteria of bv) during the menses, which resolves afterward recurrently each cycle. The steep drop off of both Progesterone and estrogen can produce an environment more hospitable to gardnerella than during the other weeks of the cycle. Get checked for BV while not on period to see if treatment w antibiotics could be indicated.