Im on birth control (implant) me and my boyfriend two days ago had sex and now i've started my period and my blood smells like fish. why?

Odor: The odor is from vaginitis or from bacteria generally found in low numbers in higher numbers. Implanon is famous for breakthrough bleeding and the extra blood may be changing the bacterial flora from the normal bacteria that creates an acidic environment to limit growth of bad smelling ones. A low dose of Estradiol .5 mg at night may help the bleeding, metronizadole for the infection. Gd luck.