Implannon

So I have the Implannon and i haven't got my period in like 5 months which is great besides the fact that there is no way to tell if I am pregnant. My boyfriend and I don't use condoms (the implant is over 99% effective) so I take pregnancy tests regularly to be sure. Does anyone know how you can track fertility so I know when I should use condoms? Or am I never fertile since I dont get my period? I dont know how this works or when pregnancy tests are even accurate since you're supposed to take them when you miss your period and like I said, I dont get mine.