nexplanon effective?

i’ve had nexplanon for almost 6 or 7 months. and my so my period is weird i’ll bleed for like an hour then it’s all brown discharge. so my period is basically gone and all. anyway, my boyfriend and i are very sexually active. he’s only cummed in me once but i’m just wondering, should i be worried about not using any extra protection? we don’t like condoms. but i’m just trying to learn to trust my birth control. how effective is isn’t? should i use back up?